Chemistry of Ground Water in the Silver 

Springs Basin, Florida, with an 

Emphasis on Nitrate

By G.G. Phelps

U.S. Department of the Interior

U.S. Geological Survey

Prepared in cooperation with:

St. Johns River Water Management District

Southwest Florida Water Management District

Marion County

Florida Department of Environmental Protection

Scientific Investigations Report 2004-5144
U.S. Department of the Interior

Gale A. Norton, Secretary

U.S. Geological Survey

Charles G. Groat, Director

U.S. Geological Survey, Reston, Virginia: 2004

For sale by U.S. Geological Survey, Information Services

Box 25286, Denver Federal Center

Denver, CO 80225

For more information about the USGS and its products:

Telephone: 1-888-ASK-USGS

World Wide Web: http://www.usgs.gov/

Any use of trade, product, or firm names in this publication is for descriptive purposes only and 

does not imply endorsement by the U.S. Government.

Although this report is in the public domain, permission must be secured from the individual 

copyright owners to reproduce any copyrighted materials contained within this report.

Suggested citation: Phelps, G.G., 2004, Chemistry of Ground Water in the Silver Springs Basin, 

Florida, with an Emphasis on Nitrate: U.S. Geological Survey Scientific Investigations Report 

2004-5144, 54 p.
 iii

Contents

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

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

Purpose and Scope. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5

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

Previous Investigations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .5

Site-Numbering System . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7

Methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7

Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .7

Hydrogeology . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8

Hydrogeologic Units and Hydraulic Properties . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9

Ground-Water Flow . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11

Silver Springs. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15

Land Use and Sources of Nitrogen. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15

Land Use, 1977 and 1995 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15

Sources of Nitrogen . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16

Land Use in the 10-Year Contributing Area. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20

Chemistry of Ground and Spring Water. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20

Major Ions and Trace Constituents . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20

Dissolved Oxygen and Nutrients . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24

Comparison to Data from 1989-90 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28

Relation of Nitrogen Concentrations to Land Use. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28

Nitrogen Isotopes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30

Wastewater Indicators . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32

Dating Spring Water using Anthropogenic Tracers . . . . . . . . . . . . . . . . . . . . . . . . . . . . 36

Summary and Conclusions. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 40

Selected References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 43

Appendix A. Water-Quality Data, 2001-2002 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 50

Appendix B. Land-Use and Geologic Codes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 54

Figures

1. Map showing approximate extent of Silver Springs basin and potentiometric 

surface of the Upper Floridan aquifer, May 1999. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3

2. Graph showing concentration of nitrate-N in water from Silver Springs, 

1955-2004 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4

3. Graphs showing annual rainfall at Ocala, Florida and discharge from Silver

Springs, 1940-2003 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6

4. Chart showing hydrogeologic units in the Silver Springs basin . . . . . . . . . . . . . . . . 8
iv 

5-7. Maps showing:

5. Generalized geology of the Silver Springs basin and well sampling 

locations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10

6. Potentiometric surface of the Upper Floridan aquifer in north-central 

Florida, May 2002, and areas contributing recharge to Silver Springs. . . . . 12

7. Potentiometric surface of the Upper Floridan aquifer in north-central 

Florida, September 2002, and areas contributing recharge to 

Silver Springs. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13

8. Diagram showing simulated volumetric flow budget for the surficial aquifer 

system, Upper Floridan aquifer, and Lower Floridan aquifer, in areas 

contributing recharge to Silver Springs based on travel times up to 

500 years, as simulated by the Peninsular Florida Model for the average 

August 1993 through July 1994 conditions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14

9. Diagram showing nitrogen cycle . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16

10. Map showing urban residential land use 1977 overlain on 1995 urban 

residential . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17

11. Graphs showing estimated annual nitrogen inputs for Marion County, 

Florida, 1940-2000 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18 

12. Maps showing 1977 and 1995 land use in the area contributing recharge 

to Silver Springs based on travel times up to 10 years . . . . . . . . . . . . . . . . . . . . . . 21

13. Boxplots showing ranges of calcium, sulfate, and nitrate-N concentrations 

for water samples collected in 1989-90 and during this study, 2001-02 . . . . . . . . 22

14. Graphs showing calcium and sulfate concentrations in water from the 

Silver Springs Main Spring, 1955-2003 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23

15. Map showing dissolved oxygen concentrations in water from the Upper 

Floridan aquifer, Silver Springs basin . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25

16. Map showing nitrate-N concentrations in water from the Upper Floridan 

aquifer, Silver Springs basin . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26

17. Graph showing nitrate-N as a function of dissolved oxygen in ground-water 

samples . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27

18. Boxplots showing total phosphorus and total orthophosphate in ground 

water, 1989-90 and 2000-01 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27

19. Boxplot showing range of 15N/14N for ground-water samples. . . . . . . . . . . . . . . . 30

20. Map showing areal distribution of 15N/14N for ground-water samples . . . . . . . . 31

21. Map showing wells where the insecticide DEET was detected in 

ground-water samples . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 35

22. Graph showing concentrations of dissolved argon and N2 in water samples 

from Silver Springs compared to equilibrium concentrations of air in water . . . 37

23. Graph showing comparison of measured tritium and tritiogenic helium 

concentrations of water samples from Silver Springs and the tritium 

concentration of rainfall at Ocala, Florida . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 39

Tables

1. Population and estimated nitrogen loads generated from wastewater in 

Marion County, 1900-2000 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6

2. Comparison of historic and current nitrate-N concentrations in filtered 

ground-water samples . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29
 v

3. Concentrations of nitrate-N in ground-water samples grouped by 1995 land 

use at well locations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29

4. Isotopic data for water samples from Silver Springs, 2001-2002 . . . . . . . . . . . . . . 32

5. Summary of compounds commonly detected in wastewater and detections 

in water samples . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 33-34

6. Dissolved gas data and summary of sulfur hexafluoride data for water 

samples collected at Silver Springs, January 29, 2002. . . . . . . . . . . . . . . . . . . . . . . 37

7. Summary of tritium-helium age data using piston-flow model for water 

samples collected at Silver Springs, January 29, 2002. . . . . . . . . . . . . . . . . . . . . . . 38

8. Concentrations of chlorofluorocarbons in water samples collected at 

Silver Springs, January 29, 2002 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 40

Conversion Factors and Vertical Datum

Multiply By To obtain

Length

inch (in.) 2.54 centimeter (cm)

foot (ft) 0.3048 meter (m)

mile (mi) 1.609 kilometer (km)

Area

square mile (mi2) 2.59 square kilometer (km2)

Volume

gallon (gal) 3.785 liter (L)

Mass

pound (lb) 0.4536 kilogram (kg)

Flow Rate

cubic foot per second (ft3/s)

 0.02832

cubic meter per second 

(m3

/s)

gallon per day (gal/d) 0.003785 cubic meter per day (m3/d)

million gallons per day 

(Mgal/d) 0.04381

cubic meter per second 

(m3

/s)

inch per year (in/yr) 25.4 millimeter per year (mm/yr)

*Transmissivity

foot squared per day (ft2/d) 0.09290 meter squared per day (m2/d)

*The standard unit for transmissivity is cubic foot per day per square foot times foot of 

aquifer thickness [(ft3/d)/ft2]ft. In this report, the mathematically reduced form, foot 

squared per day (ft2

/d), is used for convenience.

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

as follows: °C=(°F-32)/1.8.

Vertical coordinate information is referenced to the National Geodetic Vertical 

Datum of 1929 (NGVD 29).

Horizontal coordinate information (latitude-longitude) is referenced to the North 

American Datum of 1927 (NAD 27):
Abbreviations and Acronyms 

NH4

+ ammonium

Ar argon

B boron

CFCs chlorofluorocarbons

DEET N,N-diethyl-meta-toluamide

DO dissolved oxygen

FDACS Florida Department of Agriculture and 

Consumer Services

FDEP Florida Department of Environmental 

Protection

GIS geographic information system

> greater than

< less than

Mn manganese

MCL Maximum Contaminant Level

μg/L micrograms per liter

μ micron

MCU middle confining unit

MSCU middle semiconfining unit

mg/L milligrams per liter

mL milliliter

mm millimeter

NADP National Atmospheric Deposition Program

NAWQA National Water Quality Assessment

Ne neon

NO3

- nitrate

NO2

- nitrite

N nitrogen

N2 nitrogen gas

15N/14N nitrogen isotopes

OSDS onsite sewage disposal systems

P phosphorus

SF6 sulfur hexafluoride

3

Hetrit tritogenic helium or helium-3

3H/3Hetrit tritium/helium-3

TU tritium units

USGS U.S. Geological Survey

vi 
Chemistry of Ground Water in the Silver Springs Basin, 

Florida, with an Emphasis on Nitrate

By G.G. Phelps

Abstract

The Silver Springs group, in central Marion County, 

Florida, has a combined average discharge rate of 

796 cubic feet per second and forms the headwaters of the 

Silver River. The springs support a diverse ecosystem and 

are an important cultural and economic resource. Concentrations of nitrite-plus-nitrate (nitrate-N) in water from the 

Main Spring increased from less than 0.5 milligrams per 

liter (mg/L) in the 1960s to about 1.0 mg/L in 2003. The 

Upper Floridan aquifer supplies the ground water to 

support spring discharge. This aquifer is at or near land 

surface in much of the ground-water basin; nutrients 

leached at land surface can easily percolate downward into 

the aquifer. Sources of nitrogen in ground water in the 

Silver Springs basin include atmospheric deposition, fertilizers used by agricultural and urban activities, and human 

and animal wastes.

During 2000-2001, 56 wells in the area contributing 

recharge to Silver Springs were sampled for major ions, 

nutrients, and some trace constituents. Selected wells also 

were sampled for a suite of organic constituents commonly 

found in domestic and industrial wastewater and for the 

ratio of nitrogen isotopes (15N/14N) to better understand the 

sources of nitrate. Wells were selected to be representative 

of both confined and unconfined conditions of the Upper 

Floridan aquifer, as well as a variety of land-use types. 

Data from this study were compared to data collected from 

25 wells in 1989-90. Concentrations of nitrate-N in ground 

water during this study ranged from less than the detection 

limit of 0.02 to 12 mg/L, with a median of 1.2 mg/L. For 

data from 1989-90, the range was from less than 0.02 to 

3.6 mg/L, with a median of 1.04 mg/L.

Water from wells in agricultural land-use areas had 

the highest median nitrate-N concentration (1.7 mg/L), 

although it is uncertain if the 12 mg/L maximum concentration was influenced by land-use activities or proximity 

to a septic tank. The median value for all urban land-use 

areas was 1.15 mg/L. Because fewer wells were in rangeland or forested areas, those categories were grouped 

together. The median concentration for that group was 

0.09 mg/L.

The ratio of 15N/14N in ground-water samples ranged 

from -0.5 to 11.5 per mil. The median value for groundwater samples from 35 wells, 4.9 per mil, is near the top 

of the range that indicates inorganic nitrogen sources. 

In agricultural areas, the median 15N/14N was 4.8 per mil, 

indicating mostly inorganic (fertilizer) sources. In urban 

areas, the median 15N/14N was 5.4 per mil, indicating 

more influence of organic nitrogen (N) sources. Thus, in 

both agricultural and urban areas, fertilizer is an important 

inorganic source of N in ground water (and, therefore, in 

spring water as well). The influence of organic N is more 

apparent in urban areas than in agricultural areas. Two 

distinct 15N/14N values were observed in water from the 

Main Spring, one indicating an inorganic nitrogen source 

and the other indicating a mixture of sources with a strong 

influence of organic nitrogen.

 Thirty-five wells and three springs of the Silver 

Springs group (the Main Spring, the Abyss, and the Blue 

Grotto) were sampled for a suite of 63 compounds 

common in wastewater. A total of 38 compounds was 

detected, nearly all in very low concentrations. The most 

frequently detected compound was the insecticide N,Ndiethyl-meta-toluamide (DEET), which was detected in 

water from 27 wells and all three springs. The presence or 
2 Chemistry of Ground Water in the Silver Springs Basin, Florida, with an Emphasis on Nitrate

absence of DEET in ground-water samples did not seem 

to be related to land use; however, hydrogeologic conditions at the well sites (confined or unconfined) generally 

did affect the presence or absence of DEET in the ground 

water. DEET also appears to be a useful tracer for the 

presence of reused water.

Water samples were collected from the Main Spring 

and two other springs of the Silver Springs group and 

analyzed for concentrations of dissolved gasses and for 

chlorofluorocarbons (CFCs), sulfur hexafluoride, and 

tritium/helium-3 for the purpose of dating the young fraction of ground water. Apparent ground-water ages are 

based on a piston-flow model, which may not adequately 

represent a complex flow system. Apparent age for water 

from the Main Spring is about 27 years for tritium/ 

helium-3 data, compared to about 15 years based on 

sulfur hexafluoride. For the Abyss, the tritium/helium age 

is about 9 years and the sulfur hexafluoride age is about 

8 years. For the Blue Grotto, the tritium/helium-3 age is 

about 19 years, whereas the sulfur hexafluoride age is 

about 6 years. These results indicate that the flow system 

to the Abyss may be relatively shallow and rapid and 

adequately represented by the piston flow model, whereas 

the flow systems of the other springs are more complex, 

probably involving mixing with water from deeper zones 

of the aquifer. Both the nitrogen isotope and tritium/ 

helium-3 data from the Main Spring apparently indicate 

that the mixture of spring discharge from predominantly 

young to mostly older water can change rapidly and may 

not be easily related to discharge rate. CFC data could not 

be used for dating because all spring samples contained 

higher than equilibrium concentrations of CFCs from 

atmospheric sources.

All three springs are affected by the presence of 

wastewater in the contributing area, as evidenced by the 

presence of DEET and high CFC concentrations in all the 

samples. Although the conduit flow system contributing 

water to Silver Springs is complex and likely involves 

mixing of water from both shallow and deep flow zones, 

the fact that the water is relatively young (less than 30 

years) indicates the importance of minimizing nitrogen 

loading within a contributing area that has ground-water 

travel times of 30 years or less.

Introduction 

Silver Springs, in central Marion County, Florida 

(fig. 1), is the largest nontidal spring group in Florida, 

with an average discharge of 796 cubic feet per second 

(ft3

/s) for 1932-2002 (U.S. Geological Survey, 2003b).

The spring group is an important natural feature, forming 

the headwaters of the Silver River, and supporting a 

diverse ecosystem that includes numerous species of fish, 

birds, and reptiles. It also is a significant recreational, 

cultural, and economic asset for north-central Florida. 

Ground water from the Floridan aquifer system (a thick 

sequence of carbonate rocks of Eocene age) supports the 

spring discharge and is the sole source of potable water 

within the basin. The Floridan aquifer system is divided 

into the Upper and Lower Floridan aquifers, separated by 

a middle semiconfining unit. The area contributing 

recharge to the spring group, called a ground-water basin 

or springshed, includes approximately 1,200 square miles 

(mi2

) in parts of Marion, Alachua, Putnam, Lake, and 

Sumter Counties, Florida. In much of the basin, the limestone of the Floridan aquifer system is at or near land 

surface, and recharge to the aquifer occurs rapidly.

Nitrogen (N) occurs as anions in water as nitrite 

(NO2

-

) or nitrate (NO3

-

). Both species are very mobile and 

NO3

-

 is stable over a wide range of conditions (Hem, 

1985). The NO2

-

 ion is unstable in aerated water and 

generally is present in negligible concentrations. In this 

report, the combined concentration of nitrite-plus-nitrate 

reported by the laboratory is referred to as nitrate-N.

Nitrate-N concentration in the water from Silver 

Springs (and thus the concentration in Silver River) has 

increased since the mid 1950s (fig. 2). Elevated nitrate 

concentrations in surface waters may result in the growth 

of nuisance vegetation such as water hyacinth and 

hydrilla. Dense growth of such vegetation can result in 

oxygen depletion in the water beneath the vegetative mass 

due to plant death and subsequent decay (Nordlie, 1990). 

Also, increased nitrate concentrations may increase the 

growth of phytoplankton if growth is nitrogen limited, 

thus reducing water clarity.

Hydrogeologic conditions in the Upper Floridan 

aquifer, particularly the degree of aquifer confinement, 

may significantly affect the potential for nitrate enrichment of ground water. In areas where the Upper Floridan 

aquifer is unconfined—as is the case throughout most of 

the basin—nutrients leached at land surface quickly 

percolate vertically into the aquifer. In more localized 

areas where the aquifer is confined by fine-grained or 

lower-permeability sediments, although the vertical 

hydraulic gradient is downward, the lower-permeability 

sediments retard the movement of nutrient-enriched 

water. The Upper Floridan aquifer also has well-developed secondary porosity and is highly transmissive; thus, 

nutrient-enriched ground water can flow a given distance 

within a relatively short period of time (weeks to months).
Introduction 3

Ocala

Belleview

ST. JOHNS RIVER

WATER MANAGEMENT

DISTRICT

SOUTHWEST

FLORIDA

WATER

MANAGEMENT

DISTRICT

Lake

Griffin

Lake

Ocklawaha

Ockla

a

wah

e

Riv r

Lochloosa

Lake

Orange Lake

SUMTER COUNTY

MARION COUNTY

ALACHUA COUNTY

PUTNAM

COUNTY

LAKE

COUNTY

BRADFORD

COUNTY CLAY

COUNTY

CITRUS COUNTY

GROUNDWATER

BASIN

BOUNDARY

10

30

50

80

75

70

65

60

55

50

45

40

35

30

25

20

15

10

50

55

60

40

30

50

20

40

50

50

50

55

45

45

30

Rainbow

Springs

Silver

Springs

EXPLANATION

45

0 2 4 6 8 10 MILES

0 2 4 6 8 10 KILOMETERS

Base modified from U.S. Geological Survey digital data; 1:100,000, 1985

Universal Transverse Mercator projection, zone 17

STUDY AREA

BRADFORD

FOREST

PENINSULAR ARCH

OCALA UPLIFT

Figure 1. Approximate extent of Silver Springs basin and potentiometric surface of the Upper Floridan aquifer, 

May 1999 (from Bradner and Knowles, 1999).
4 Chemistry of Ground Water in the Silver Springs Basin, Florida, with an Emphasis on Nitrate

For example, Knochenmus (1967) reported a dye-trace 

travel time in the Ocala area of 1.3 miles in less than 

1 week. Because the basin generally is internally drained, 

most of the water that recharges the aquifer eventually is 

discharged either by pumping wells or at Silver Springs.

Land-use activities within the Silver Springs basin in 

Marion County historically have been predominantly 

agricultural. However, the City of Ocala continues to 

experience rapid population growth, as do surrounding 

rural areas that have become increasingly attractive for 

development as retirement communities. These communities often include golf courses and residential areas with 

turf that require fertilization. Of particular concern is the 

potential for nutrients leached from these fertilizers to 

enter the Upper Floridan aquifer. In addition, in 1990, 

only about 35 percent of dwellings in Marion County 

were served by public sewage treatment facilities; the 

remainder presumably use onsite sewage disposal 

systems (OSDS), usually called septic tanks (U.S. Census 

Bureau, 2002).

Nitrate concentrations greater than 10 milligrams per 

liter (mg/L) present a health hazard (primarily for infants) 

and exceed the primary drinking-water standard established by the U.S. Environmental Protection Agency 

(2003). In 1994, nitrate concentrations as high as 5.2 

mg/L were observed in ground water in the adjacent Rainbow Springs basin (Jones and others, 1996, appendix II). 

Results of sampling 20 wells in the Silver Springs basin 

in 1989-90 indicated a range of nitrate-N concentrations 

from less than 0.02 to 3.6 mg/L, with a median of 

1.2 mg/L (Phelps, 1994). In the decade between that 

study and the one described in this report, only three wells 

in the basin sampled previously (as part of the Florida 

Department of Environmental Protection (FDEP) Very 

Intensely Studied Area network) were resampled.

To assess the possible effects of past and present landuse activities on water quality in the Silver Springs basin, 

and thus, the potential for future land-use changes to 

degrade ground-water quality, existing chemical constituent concentrations in ground water need to be documented. 

Also, an assessment is needed of how nutrient concentrations, particularly nitrate (because of its mobility in groundwater systems), may vary as a function of localized hydrogeologic conditions (that is, unconfined or semiconfined 

aquifer) as well as with land use. A basin-wide evaluation 

of the occurrence and distribution of nitrate in the Silver 

Springs basin will enhance understanding of the processes 

by which nutrients enter and move through the groundwater flow system. The results will help area planners and 

water-resource managers formulate recommendations for 

design of new developments, and thus minimize the potential for ground water to become contaminated from changes 

to existing land-use activities. Also, by providing a baseline 

of current conditions, managers will be better able to quantify changes in ground-water chemistry that may occur with 

future development. Results may also be useful in helping 

planners identify areas, based on differences in hydrogeologic conditions, where ground-water quality is most (or 

least) likely to be affected by development. Finally, the 

results will provide useful information for designing more 

effective monitoring-well networks needed to identify 

changes in ground-water quality in the future. To meet these 

objectives, the U.S. Geological Survey (USGS), in cooperation with the St. Johns River Water Management District 

(SJRWMD), the Southwest Florida Water Management 

District (SFWMD), Marion County, and FDEP, began a 4-

year study in 1999 to document the chemistry of ground 

water in the Silver Springs basin, with an emphasis on the 

occurrence and distribution of nitrate-N and to relate, if 

possible, nitrate occurrence and concentrations with current 

and past land-use activities and selected hydrogeologic 

conditions.

Figure 2. Concentration of nitrate-N in water from 

Silver Springs, 1955-2004.
Introduction 5

Purpose and Scope

This report presents the results of sampling 56 wells 

and three springs of the Silver Springs group for major 

and selected trace constituents, nutrients and field 

measurements of temperature, pH, specific conductance, 

and dissolved oxygen. Selected wells and the three 

springs also were sampled for nitrogen isotopes (15N/14N) 

and a suite of organic constituents commonly found in 

domestic and industrial wastewater. The springs were 

sampled for chlorofluorocarbons (CFCs), sulfur hexafluoride, dissolved gasses, and the ratio of tritium to 

helium-3 (3

H/3Hetrit) to provide information for dating the 

spring water. Although the study area includes the entire 

ground-water basin, emphasis was placed on the areas 

relatively close to the spring group and also on the southern half of Marion County, where the most rapid development activities are expected to occur. Land-use 

information for 1977 and 1995 were compiled into 

geographic information system (GIS) coverages so that 

the effects of land use on ground-water chemistry could 

be evaluated. Estimates were made of historic nitrogen 

loading to the basin by compiling fertilizer sales and 

recommended application data, atmospheric deposition 

data, rates of nitrogen production by farm animals, and 

loads from wastewater. By adding particle tracking to a 

ground-water flow model (Sepúlveda, 2002), areas 

contributing recharge to Silver Springs for various travel 

times were delineated.

Description of Study Area

The Silver Springs ground-water basin, as delineated 

on the basis of the potentiometric surface of the Upper 

Floridan aquifer in May 1999 (Bradner and Knowles, 

1999), encompasses about 1,200 mi2

 in north-central 

Florida (fig. 1), although fluctuations in the altitude of the 

potentiometric surface can result in minor variations in 

the size of the basin. The potentiometric surface of May 

1999 generally represents conditions at the end of the dry 

season. The gently rolling topography results from the 

combination of karst depressions caused by dissolution of 

limestone and hills capped with less permeable sediments 

that retard dissolution. Land-surface altitudes range from 

about 65 to 180 feet (ft) above NGVD 29. The area is 

characterized by an almost complete absence of surface 

drainage. Most of the drainage is internal, either directly 

into closed depressions or by seepage into the unconfined 

limestone of the Upper Floridan aquifer. Ground-water 

basins in the area do not coincide with the boundaries of 

surface-water drainage divides (Faulkner, 1973).

Climate of the area is humid subtropical, with hot, 

rainy summers and cool, generally dry winters. Annual 

rainfall averages about 54 inches per year (in/yr) and the 

average air temperature is about 71 o

F (National Oceanic 

and Atmospheric Administration, 2003). A drought in 

2000-2002 resulted in the lowest flow measurement on 

record for the Silver Springs group, 350 ft3

/s in June 2001 

(fig. 3). The average flow for 1932-2002 was about 

790 ft3

/s.

Population in Marion County, as in many parts of 

Florida, has increased significantly (table 1). From 1970-

2000, the population increased about 275 percent. Marion 

County has changed from predominantly rural to a 

mixture of urban-suburban-rural land uses. The growth of 

Florida as a retirement destination has resulted in the 

development of numerous subdivisions, many of which 

include golf courses. Areas that once were thoroughbred 

horse farms were redeveloped as retirement communities 

or shopping areas.

Previous Investigations

Previous studies of the ground-water resources of 

Marion County and the Silver Springs area include three 

investigations focused on the potential effects of the 

proposed Cross-Florida Barge Canal: Knochenmus 

(1967) investigated ground-water travel times by using 

dye traces; Faulkner (1973) conducted a study of the 

stratigraphy, structural geology, ground-water flow 

system, and ground-water chemistry in central Marion 

County; and Tibbals (1975) conducted aquifer tests in the 

Upper Floridan aquifer in the proposed canal right-ofway. Information about drainage wells in Ocala was 

collected by Kimrey and Fayard (1984); a report by Environmental Resources Management-South, Inc., (1988) 

documented contamination of the Upper Floridan aquifer 

in downtown Ocala. The hydrogeology, ground-water 

quality, and potential for contamination of the Upper 

Floridan aquifer in the Silver Springs basin were 

described by Phelps (1994). Ground-water flow conditions in the adjacent Rainbow Springs basin were 

described by Geo-Trans, Inc., (1988). Evapotranspiration 

rates in both the Silver Springs and Rainbow Springs 

basins were estimated by Knowles (1996). A detailed 

analysis of the sources of nitrate in water discharging 

from Rainbow Springs was presented by Jones and others 

(1996). Isotope concentrations of water from Silver 

Springs and other springs in Marion County were reported 

by Toth (1999 and 2003).
6 Chemistry of Ground Water in the Silver Springs Basin, Florida, with an Emphasis on Nitrate

Figure 3. Annual rainfall at Ocala, Florida, and discharge from Silver Springs, 1940-2003.

Table 1. Population and estimated nitrogen loads generated from wastewater in Marion County, Florida, 1900-2000.

[Source of population numbers: U.S. Census Bureau, 2002. Nitrogen loads in kilograms per year, based on a per capita estimate of 4.09 kilograms per year from 

Otis and others, 1993]

Year 1900 1910 1920 1930 1940 1950 1960 1970 1980 1990 2000

Total population 24,403 26,941 23,968 29,578 31,243 38,187 51,616 69,030 122,488 194,833 258,916

Nitrogen loads 99,808 110,189 98,029 120,974 127,784 156,185 211,109 282,333 500,976 796,867 1,058,966
Introduction 7

Site-Numbering System

The USGS National Water Data Information System 

uses a 15-digit number (site identification number) based 

on latitude and longitude to identify wells. The first six 

digits denote the degrees, minutes, and seconds of latitude; the next seven digits denote the degrees, minutes, 

and seconds of longitude; and the last two digits denote a 

sequential number for a site within a one-second grid. 

Well site identification numbers generally end in “01” or 

“02.” For convenience, in this report wells also are given 

a sequential well number (listed in app. A) that refers to 

well locations shown in the figures. Surface-water sites 

that are part of the USGS data-collection network are 

identified by an 8-digit downstream order number. The 

8-digit site identification number for Silver Springs 

(Main Spring) is 02329500. Miscellaneous surface-water 

quality data-collection sites generally are assigned 15-

digit numbers based on latitude and longitude and ending 

in sequence number “00.”

Methods

During this study, water samples were collected from 

56 wells and three springs of the Silver Springs group. 

Most of the wells were in regular use and equipped with 

a dedicated pump. A few wells did not have pumps and 

were purged and sampled with a portable electric 

submersible pump. Wells were purged until at least three 

casing volumes had been removed and until temperature, 

pH, and specific conductance had stabilized. Water 

samples for major ions and nutrients were collected 

according to protocols described by the U.S. Geological 

Survey (1997-2003) and analyzed at the USGS laboratory 

in Ocala, Fla. Samples analyzed for nutrients were kept in 

the field at 4 oC before being taken to the lab. Water 

samples for anion and cation analysis were filtered by 

using 0.45-micron (μ) Gelman capsule filters. Cation and 

trace metal samples were collected in acid-washed bottles 

and acidified with 70 percent nitric acid.

Isotopic values were reported with the standard delta 

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

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

For δ15N, R= 15N/14N. Samples for nitrogen isotope analysis were filtered and collected in 1-liter (L) brown plastic 

bottles, kept chilled at 4 o

C, and shipped by the USGS 

central laboratory in Denver, Colo., to a contract laboratory. Nitrite-plus-nitrate was extracted by using Devarda’s 

reduction and δ15N was determined by using mass spectrometric analysis with analytical uncertainties of about 

+0.2 per mil.

Samples from springs were collected by lowering a 

positive displacement dual-piston (Bennett) pump head 

from the water surface down to the spring vent. Water was 

pumped through 0.63-cm diameter copper tubing (refrigerator grade). Water samples for 3

H/3Hetrit analyses were 

collected in pinch-off copper tubes (10 mm diameter, 80 

cm length, approximately 40 mL volume) while applying 

back pressure to the discharge from the sample tube to 

prevent formations of gas bubbles during sample collection. These samples were analyzed at the Noble Gas Laboratory of Lamont-Doherty Earth Observatory at Columbia 

University using quantitative gas extraction by mass spectrometric techniques (Schlosser and others, 1989) and 

Ludin and others (1998). Water samples for CFC analysis 

were collected in glass ampules that were sealed in the 

field. Water samples for dissolved gas analysis were 

collected in septum glass vials; samples for sulfur 

hexafluoride (SF6) analysis were collected in glass bottles 

and shipped to the USGS CFC laboratory in Reston, Va. 

Both CFC and SF6 concentrations are determined by 

GC-ECD (gas chromatography with electron capture 

detection) (Busenberg and Plummer, 1992 and 2000).

Water samples for analysis of organic constituents 

commonly found in domestic and industrial wastes were 

collected in 1-L amber glass bottles which had been baked 

at 350 oC. The samples were chilled at 4 oC, and shipped 

to the USGS Central Laboratory in Denver, Colo., where 

they were filtered through a 0.7-μ glass fiber filter before 

extraction. For quality assurance purposes, replicate 

samples and field blank samples were collected during 

each sampling trip. Because compounds such as caffeine 

and insecticides were included in the analysis, field 

personnel neither consumed caffeine nor used personal 

insecticides on sampling days. Samples were analyzed by 

using gas chromatography and mass spectrometry, as 

described by Zaugg and others (2002).

Acknowledgments

The author is grateful to Julian Wayland (USGS, 

Reston, Va.) and Gen Phelps, USGS Volunteer for 

Science, for assistance with field data collection. Helpful 

discussions and comments were provided by Brian Katz 

(USGS, Tallahassee, Fla.), Amy Swancar (USGS, Tampa, 

Fla.), and L. Niel Plummer, (USGS, Reston, Va.).
8 Chemistry of Ground Water in the Silver Springs Basin, Florida, with an Emphasis on Nitrate

Hydrogeology 

The hydrogeology of the Silver Springs basin was 

described in detail by Faulkner (1973). The Florida 

Plateau is a large, tectonically stable platform of carbonate rocks that overlie the post-Paleozoic Coastal Plain 

floor. In central Marion County, the sedimentary rocks 

are about 4,000 ft thick of which about 1,500 to 2,500 ft 

are Tertiary age and younger. The geologic formations of 

interest in the study area are the Avon Park Formation, the 

Ocala Limestone, the Hawthorn Group, and unconsolidated post-Miocene sediments (fig. 4). The Avon Park 

Formation, of middle Eocene age, is composed of alternating layers of hard to very hard dark-brown dolomite 

and softer, light-brown to tan limestone. The Avon Park 

Formation is highly fractured and contains cavities. The 

Ocala Limestone is composed of soft white to cream to tan 

limestone which usually is fossiliferous, with occasional 

inclusions of chert. An erosional unconformity separates 

Figure 4. Hydrogeologic units in the Silver Springs basin (from Faulkner, 1973, and Scott and others, 2001).
Hydrogeology 9

the Avon Park Formation from the overlying Ocala Limestone. In some parts of Marion County, the Ocala Limestone has been substantially eroded and the Avon Park 

Formation occurs near land surface.

The Hawthorn Group of Miocene age unconformably overlies the Ocala Limestone and, where present, 

ranges in thickness from a few feet to more than 100 ft in 

the eastern part of Marion County. Hawthorn Group sediments consist mostly of quartz sand interbedded with 

clay, sandy phosphatic clay, and hard, dense limestone or 

dolostone. The stratigraphy and occurrence of the 

Hawthorn Group was described by Scott (1988). In much 

of western Marion County, the Hawthorn Group has been 

eroded; the remaining deposits commonly form caps on 

hilltops. East of the Ocklawaha River, the Hawthorn is 

present as a continuous layer; the resulting expression at 

land surface is a change from rolling karst hills in the 

western part of the County to a more flat, often poorly 

drained landform. Overlying the Hawthorn Group in parts 

of Marion County are a variety of mostly clastic deposits 

that range in age from Pliocene to Holocene (fig. 5) and 

in thickness from 0 to about 100 ft. In most of the Silver 

Springs ground-water basin, the thickness of sediments 

overlying the Ocala Limestone ranges from 10 to 50 ft.

Two regional structural features are important in the 

study area: the Peninsular Arch and the Ocala Uplift 

(fig. 1). The Peninsular Arch is the primary structural 

control for sediments deposited during Cretaceous and 

early Tertiary time (Faulkner, 1973). The axis of the 

Peninsular Arch runs from northwest to southeast through 

eastern Marion County. Later in geologic time, another 

smaller uplift occurred to the west of the Peninsular Arch, 

causing upwarping of lower Tertiary rocks. The axis of 

the latter, the Ocala Uplift, runs roughly parallel to and to 

the west of the Peninsular Arch. As the Ocala Uplift 

developed, tensional stresses caused the formation of 

fractures and possibly some normal faults in the Tertiary 

rocks (Miller, 1986). Faulkner (1973) mapped two conjugate sets of fractures in the study area that intersect at 

right angles, based on interpretation of aerial photographs, and reported that most of the numerous cave 

systems in Marion County are oriented along one or the 

other of the fracture systems. Fracture systems are apparent in the limestone near land surface and also at depth, 

based on interpretations of well logs (Phelps, 1994). Flow 

of carbon dioxide-rich recharge water through the fracture systems has resulted in the dissolution of rock and 

enlargement of fractures, which are the easiest flow paths 

for water. This process has been ongoing throughout the 

geologic past and continues to occur.

Hydrogeologic Units and Hydraulic Properties

In parts of the study area, the surficial aquifer system 

and the intermediate confining unit, which generally 

correspond to the post-Miocene and Miocene sediments, 

respectively (fig. 4), overlie the Floridan aquifer system. 

Where present (generally in the eastern part of Marion 

County or in outliers of the Hawthorn Group sediments in 

the western part of the County, fig. 5), these hydrogeologic units are thin and discontinuous. Because of its 

discontinuity, the surficial aquifer system is not a significant source of water in the study area.

The principal hydrogeologic unit of interest in the 

study area is the Upper Floridan aquifer, which generally 

is about 300 ft thick in the study area (Sepúlveda, 2002, 

fig. 7) and includes the upper part of the Avon Park 

Formation, and where present, the Ocala Limestone. 

Porosity of the Upper Floridan aquifer in central Florida 

ranges from 15 to 40 percent, based on thin sections of 

five samples (Thayer and Miller, 1984). Because of the 

significant porosity of the limestone, flow in the Upper 

Floridan aquifer occurs in both rock matrix and the fracture or conduit systems, unlike the karst in older limestone 

in other geologic settings, where virtually all of the flow 

occurs in the conduits.

Transmissivity of the Upper Floridan aquifer depends 

on both primary (matrix) and secondary (conduit) porosity of the aquifer, and therefore, can vary widely. Three 

aquifer tests of the upper 100 ft of the Upper Floridan 

aquifer yielded transmissivity ranging from 6,200 to 

29,500 feet squared per day (ft2

/d) (Tibbals, 1975). Transmissivity calculated from flow nets represents the full 

thickness of the aquifer and includes the effects of large 

solution channels—values ranged from 10,700 to 

25,500,000 ft2

/d, with an average value of 2,000,000 ft2/d 

(Faulkner, 1973). Transmissivity for the Upper Floridan 

aquifer near Silver Springs from a calibrated model of 

ground-water flow was 12,000,000 ft2

/d (Sepúlveda, 

2002). The large transmissivities result in rapid groundwater transit times; an average ground-water velocity of 

0.8 ft per minute at a gradient of about 1 ft per mile was 

reported by Knochenmus (1967), based on a dye trace in 

the Silver Springs basin.

Beneath the Upper Floridan aquifer lie the middle 

semiconfining unit and the Lower Floridan aquifer. The 

middle semiconfining unit (MSCU) generally is about 

100 ft thick in the study area (Sepúlveda, 2002, figs. 7 and 

8). It is composed of soft, fine-grained limestone and 

dolomitic limestone and occurs in the lower part of the 

Avon Park Formation (Miller, 1986). To the west of the 

study area, the MSCU is not present; in that area, the 

Upper and Lower Floridan aquifers are separated by the
10 Chemistry of Ground Water in the Silver Springs Basin, Florida, with an Emphasis on Nitrate

Figure 5. Generalized geology of the Silver Springs basin and well sampling locations (geology from 

Scott and others, 2001).
Hydrogeology 11

middle confining unit (MCU), composed of gypsiferous 

dolomite and dolomitic limestone of considerably lower 

permeability than the MSCU (Sepúlveda, 2002; Miller, 

1986). The horizontal facies change between the MSCU 

and the MCU is not well defined; however, lateral gradation between the two units could account for the increased 

sulfate concentrations with depth in ground-water 

samples reported by Faulkner (1973). Beneath the 

MSCU, at an altitude of about 400 ft below NGVD 29, is 

the top of the Lower Floridan aquifer. Lithologic data for 

this aquifer in Marion County could not be located; few, 

if any, wells penetrate the aquifer in the County because 

adequate water supplies can be obtained from shallower 

formations.

All of the water that discharges from Silver Springs 

originates as recharge to the aquifer system in northcentral Florida. The average flow from the springs is 

about 800 ft3

/s or about 12 inches per year (in/yr) 

(Knowles, 1996). The discharge from wells in Marion 

County was estimated to be 71 million gallons per day 

(Mgal/d) or 110 ft3

/s (about 1.6 inches (in.)) in 2000 

(U.S. Geological Survey, 2003a). This total discharge is 

supported by recharge rates ranging from about 10 to 

20 in/yr. Such recharge rates are in agreement with results 

from computer modeling (Sepúlveda, 2002). High rates 

of recharge occur in areas where the Upper Floridan aquifer is at or near land surface or covered by only a thin 

semiconfining layer (fig. 5). The aquifer is unconfined in 

areas where the Ocala Limestone crops out and poorly to 

moderately confined in areas where sediments of the 

Hawthorn Group occur at land surface. In the eastern part 

of the County, where Pliocene and younger sediments 

crop out, the Upper Floridan aquifer is confined. The area 

along the Ocklawaha River is a discharge area for the 

Upper Floridan aquifer.

Ground-Water Flow

Delineation of the ground-water basin, sometimes 

called the springshed, historically has been based on the 

potentiometric surface of the Upper Floridan aquifer by 

connecting points perpendicular to the potentiometric 

contours. The boundaries of the basin can change over 

time as the potentiometric surface fluctuates. For example, the area of the adjacent Rainbow Springs basin 

ranged from about 645 mi2

 in May 1994 to 772 mi2 in 

September 1994 (Jones and others, 1996). Seasonal fluctuations of water levels in wells in the Silver Springs 

basin generally are about 3 to 5 ft, but the water level 

change in any particular well can vary depending on local 

rainfall and pumpage rates. In an area where the transmissivity is high, the hydraulic gradient is very flat and 

changes in the potentiometric surface may be small and 

the basin divide difficult to define. This is true of the flow 

divide between the Silver Springs and Rainbow Springs 

basins, along the western boundary of the Silver Springs 

basin. During this study, wells were added to the USGS 

potentiometric-surface network in May and September 

2002 to better delineate the divide and help explain the 

effects of potentiometric fluctuations (figs. 6-7). These 

additions did not greatly improve delineation of the Silver 

Springs basin; it is likely that many more wells are needed 

to define the divide more accurately based on the potentiometric surface. Therefore, another method was used to 

delineate the ground-water basin for Silver Springs.

By adding particle tracking to a calibrated model of 

ground-water flow, the area contributing recharge to a 

spring for a particular range of travel times can be delineated (Shoemaker and others, 2003). The area contributing recharge to Silver Springs for water particles having 

travel times up to 500 years was computed with a flow 

model of peninsular Florida (Sepúlveda, 2002) and is 

shown in figures 6-7. The method used to perform the 

particle-tracking is described by Shoemaker and others 

(2003). The 500-year area encompasses about 570 mi2

and extends farther south and not as far west or north as 

the contributing area based on the potentiometric surface. 

The area contributing recharge excludes discharge areas 

of the Upper Floridan aquifer along the Ocklawaha River 

that were included in the basin delineation based on the 

potentiometric surface. The time period for which this 

model was calibrated (August 1993-July 1994) was 

slightly drier than normal, as evidenced by the fact that 

discharge from Silver Springs was about 620 ft3

/s (compared to the average for the period of record (1932-2002), 

796 ft3

/s). The size and extent of the contributing area for 

other climatic conditions are likely to be different.

The area contributing recharge to Silver Springs for 

travel times up to 10 years also was computed for the 

same calibration period in order to allow an evaluation of 

water chemistry and land-use data on a time scale of interest to water managers. In general, models may indicate 

different contributing areas because of different hydrologic stresses, subsurface permeability properties, and 

boundary conditions used to calibrate each model 

(Shoemaker and others, 2003). In particular, travel times 

are inversely related to aquifer porosity values used in 

the models. Given the range of porosity in the study 

area, travel times could vary by an order of magnitude.
12 Chemistry of Ground Water in the Silver Springs Basin, Florida, with an Emphasis on Nitrate

Figure 6. Potentiometric surface of the Upper Floridan aquifer in north-central Florida, May 2002, 

(modified from Kinnaman and Knowles, 2002) and areas contributing recharge to Silver Springs.
Hydrogeology 13

Figure 7. Potentiometric surface of the Upper Floridan aquifer in north-central Florida, September 2002, 

(modified from Kinnaman and Knowles, 2003) and areas contributing recharge to Silver Springs.
14 Chemistry of Ground Water in the Silver Springs Basin, Florida, with an Emphasis on Nitrate

The effect of this variation would be that the contributing 

area delineated would be for a somewhat longer travel 

time (perhaps 20 years rather than 10). A detailed review 

of existing ground-water flow models and, probably, 

refinements of those models would be needed to delineate 

a contributing area (springshed) that might be used by 

water managers for regulatory purposes.

The relative proportion of springflow recharged 

within particular contributing areas also is of interest for 

constructing geochemical mixing models. Based on the 

flow model for peninsular Florida (Sepúlveda, 2002), 

about 17 percent of the flow to Silver Springs is 

recharged within the 10-year contributing area and about 

48 percent within the 30-year contributing area (Shoemaker and others, 2003); these rates are 109 ft3

/s and 

307 ft3

/s, respectively, for the August 1993-July 1994 

calibration period of the model.

A ground-water flow model also can be used to estimate a water budget for the study area. Sepúlveda (2002) 

determined that the time period August 1993-July 1994 

represented steady-state conditions (negligible change in 

ground-water storage) in the Floridan aquifer system in 

peninsular Florida, based on evaluations of water levels. 

The flow model calibrated for that period was used to 

compute a water budget for the area contributing recharge 

to Silver Springs based on water travel times up to 500 

years (fig. 8). From August 1993-July 1994, withdrawals 

from wells in the area totaled about 1.36 in. Recharge 

through drainage wells was about 0.12 in.; net recharge to 

the Upper Floridan aquifer was about 12.9 in. The net 

recharge rates exclude water lost to evapotranspiration, 

which was estimated to range from about 29 to 49 in/yr for 

the Silver Springs basin, based on data from 1965-94 

(Knowles, 1996).

Figure 8. Simulated volumetric flow budget for the surficial aquifer system, Upper Floridan aquifer, and Lower Floridan 

aquifer, in areas contributing recharge to Silver Springs based on travel times up to 500 years, as simulated by the 

Peninsular Florida Model for the average August 1993 through July 1994 conditions (N. Sepúlveda, U.S. Geological 

Survey, written commun., 2003).
Land Use and Sources of Nitrogen 15

Silver Springs

Silver Springs consists of several large springs—

including the Main Spring, also called Mammoth Spring; 

the Abyss, also known as Reception Hall; and the Blue 

Grotto—and numerous other smaller springs (sand boils) 

along the edges of the spring run. The springs form the 

headwaters of the Silver River, which flows eastward a 

few miles to the Ocklawaha River (fig. 1). Faulkner 

(1973) concluded that the locations of Silver Springs and 

the Ocklawaha River are related to the regional fracture 

systems in the rocks that compose the Floridan aquifer 

system. Faulkner suggested that structural lowering of the 

river valley and the area to the east of the river resulted in 

eastward flow of ground water in the limestone being 

blocked by lower permeability sediments of the 

Hawthorn Group (which were substantially eroded from 

areas west of the river). The ground water surfaced to 

form Silver Springs. The Main Spring contains a cave 

system of passages, some of which are interconnected, 

that likely are controlled by the regional fracture systems, 

based on a survey of about 2,260 ft of the system by 

divers (E. Hutcheson, private citizen, Ocala, Fla., written 

commun., 1993).

The flow system contributing water to Silver Springs 

is relatively shallow, based on hydrologic and waterchemistry data reported by Faulkner (1973). Faulkner 

concluded that hydrologic indicators such as the shape 

of the potentiometric surface and the results of aquifer 

tests and dewatering operations in the Barge Canal area 

suggested that the permeability of the Avon Park Formation was considerably lower than that of the Ocala Limestone. This probably indicates that most of the flow to 

the springs occurs in the Ocala Limestone rather than in 

deeper zones. In the study area, the Ocala Limestone 

generally is about 100 ft thick, implying that most flow 

to Silver Springs occurs in the upper 100 ft of the aquifer. 

Faulkner (1973) also discussed dissolved solids and 

sulfate concentration data and temperature data, all of 

which point to a shallow flow system. Based on these 

data, Faulkner (1973) estimated that about 86 percent of 

the water discharging from Silver Springs is from the 

upper part of the aquifer and about 14 percent is from the 

lower part or the Avon Park Formation.

Land Use and Sources Of Nitrogen 

Land-use practices are generally thought to affect the 

chemical quality of ground water, although such effects 

may not be easy to quantify. This is true of nitrogen, 

which has natural and anthropogenic sources and is 

cycled continually in the environment by a complex set of 

chemical processes. The competing processes of 

(1) mineralization and immobilization and (2) nitrification and denitrification occur simultaneously (fig. 9). 

Some of these processes are mediated by microorganisms. 

Nitrate, which is highly soluble and mobile, is perhaps the 

most widespread contaminant of ground water (Hallberg 

and Keeney, 1993). It is an important plant nutrient and 

also is a component of all proteins. Atmospheric deposition (both by rainfall and dryfall) contributes nitrogen to 

an ecosystem. Most natural systems maintain a balance 

between the atmospheric and soil-plant systems (Hallberg 

and Keeney, 1993), although the amount of nitrogen 

added to the atmosphere by industrial processes has 

increased steadily since the 1940s (Stevenson, 1982). 

Other sources of nitrogen include fertilizer and animal and 

human wastes. The relative amounts of nitrogen from 

these sources and their potential contributions to NO3

-

 in 

ground water will be discussed in the following section.

Land Use, 1977 and 1995

 GIS coverages of land use for 1977 (U.S. Geological 

Survey, 1990) and 1995 (St. Johns River Water Management District, 1999) were analyzed to better understand 

the relations between land use and ground-water chemistry. 

Some difficulties were encountered, however, in relating 

water chemistry to land use and in comparing land-use 

data for different time periods. The land use that is indicated for a particular well site may be significantly different from adjacent land use. This can be especially 

problematic in a karstic area if a different land-use activity 

occurs upgradient from a sample site. Also, land-use categorization can be somewhat subjective and may not accurately reflect the actual activities at the site. For example, 

land use at one well sampled was indicated as pasture. 

After field verification, it was determined that the well is 

situated in a grassy field next to a mobile-home park that 

uses septic tanks for wastewater disposal. The site probably is more representative of urban residential than 

pasture land use. Finally, in an area that is undergoing 

rapid growth, such as Marion County, land-use coverages 

rapidly become out-of-date.

Comparing land-use coverages for 1977 and 1995 is 

difficult because of changes in the methods used for creating the coverages. In 1995, for example, the urban residential land category was divided into low, medium, and 

high density urban residential, depending on the number 

of dwellings per acre; but in 1977 these distinctions were 

not made. Subcategories for agricultural, range, and forest 

lands also were introduced in the 1995 coverage. Given 
16 Chemistry of Ground Water in the Silver Springs Basin, Florida, with an Emphasis on Nitrate

the discrepancies, perhaps the most useful comparison 

between the two time periods is the increase of urban 

land. Estimated urban residential land use in the springshed was about 38 mi2

 in 1977 and increased to about 

164 mi2

 in 1995 (fig. 10).

Sources of Nitrogen

A nitrogen loading history for Marion County was 

estimated by compiling the inputs of nitrogen from various sources such as atmospheric deposition, fertilizers, 

and animal and human waste (fig. 11). These estimates 

are given in kilograms of total nitrogen, although not all 

of the nitrogen was in the form of nitrate. Also, the loads 

(except for atmospheric deposition) are for the entire 

County and were not apportioned between the Silver 

Springs basin and the other major springshed in Marion 

County, the Rainbow Springs basin. About 260 mi2

 of the 

Rainbow Springs basin lie in Marion County, compared to 

about 790 mi2 of the Silver Springs basin.

Atmospheric deposition of nitrogen was estimated 

from data collected at a National Atmospheric Monitoring 

Deposition Program (NADP) (2002) site in Bradford 

Forest, Bradford County (fig. 1). Some of the nitrogen in 

the atmosphere is naturally occurring but atmospheric 

deposition of nitrogen probably has increased tenfold 

since the late 19th century (Howarth and Rielinger, 2004),

(D)

(C)

(B)

(A)

EXPLANATION

ANIMAL PROTEIN

PLANT PROTEIN

PLANT GROWTH

BY BACTERIA

BACTERIAL ACTION

NITRATES

AND

NITRITES

(transient)

(B)

AMINO ACIDS,

AMMONIA, UREA

(D)

N fixation is any process by which elemental N2 reacts to form a compund.

DENTRIFICA

(C)

TION

ORGANIC RESIDUES

DECAY

MINERALIZATION

IM

MOBILIZATION

Figure 9. Nitrogen cycle (modified from Keenan and Wood, 1971; Sumner and Bradner, 1996; and Pidwirny, 2003).
Land Use and Sources of Nitrogen 17

Figure 10. Urban residential land use in 1977 overlain on 1995 urban residential.
18 Chemistry of Ground Water in the Silver Springs Basin, Florida, with an Emphasis on Nitrate

Figure 11. Estimated annual nitrogen inputs for Marion County, Florida, 1940-2000.
Land Use and Sources of Nitrogen 19

mostly from the burning of fossil fuels and from vehicle 

emissions. The NADP station began operating in 1978, 

although data for that year are not included because there 

is only a partial record. The nitrogen load was calculated 

based on the methodology described by Katz and others 

(1999) and assuming that the basin size was 1,200 mi2

. 

The loading from atmospheric nitrogen fluctuates with 

time but does not show a trend. The amount of nitrogen 

from atmospheric deposition that reaches ground water is 

largely unknown. At a study site in the United Kingdom, 

it was estimated about 11 to 18 percent of the total nitrogen leached to the ground water came from atmospheric 

deposition (Harris, 2004). In many forested regions, only 

10 to 20 percent of atmospherically deposited nitrogen is 

exported downstream in the surface watershed (Howarth 

and Rielinger, 2004). To reflect the uncertainty in loading 

from atmospheric deposition, the relative loads—assuming 

that 10 and 20 percent of the atmospheric nitrogen 

reaches the ground water—also are shown in figure 11.

Nitrogen loads from fertilizers can be difficult to estimate because data are compiled on a county-wide basis. 

Fertilizer sales data, compiled annually by the Florida 

Department of Agriculture and Consumer Services 

(FDACS), tend to overestimate loads because those data 

include fertilizer sold in Marion County by large distributors but transported to other counties for use. The amount 

of each type of fertilizer material sold was multiplied by the 

percentage of N it contained and summed to obtain annual 

estimates of N from fertilizers sold in the County (Katz and 

others, 1999). Another method for estimating nitrogen 

loads is to multiply recommended fertilizer application 

rates by the estimated number of acres of various crops 

from agricultural census data. The number of acres cultivated in various crops was estimated from the Florida Agricultural Census (U.S. Department of Agriculture, 1997; 

U.S. Department of Agriculture, unpublished data) and 

multiplied by the respective recommended application rate 

for various crops compiled by the Florida Institute of Food 

and Agricultural Sciences (Jones and others, 1974; Kidder 

and others, 1998). Implicit in this calculation is the 

assumption that growers are applying fertilizer at the 

recommended rate. Fertilizer sales and recommended 

application information begin in the 1950s. Low N prices 

in the 1940s and 1950s and the desire to improve citrus 

crop yields resulted in high application rates during that 

period. Higher prices since the early 1970s influenced 

growers to reduce the rates of N applications (Stricker and 

Muraro, 1981). Recommended fertilizer use estimates also 

are county-wide so it was not possible to determine what 

percentage of the fertilizer applied was used in Marion 

County but outside the Silver Springs basin. The actual 

nitrogen load from agriculture in the springshed probably is 

bracketed by the sales and recommended application rate 

estimates.

In addition to agricultural uses, fertilizers also are 

applied to residential turfgrass and to golf courses. Estimating such loads may be difficult because some of the 

fertilizer already may be accounted for in the county-wide 

sales data. For comparison with other estimates of fertilizer use, the golf course load for 2000 was estimated 

based on about 20 golf courses in the study area that year. 

Based on an approximate application rate of 28 tons 

(25,000 kilograms) (kg) per year of N per golf course 

(Jones and others, 1996), the total N load was about 

500,000 kg (fig. 11). Residential fertilizer use in the entire 

1,200 mi2

 study area for 1977 and 1995 (the years for 

which land-use coverage data are available) was calculated for comparison with other N load estimates. In 1995, 

urban residential land was subdivided into low, medium, 

and high density, whereas in 1977 all urban residential 

land was combined. Percentages of low, medium, and 

high density were assumed to be the same in 1977 as in 

1995, which may not be the case. For example, in 1995 

about 91 percent of the urban residential land was low 

density, about 3 percent medium density, and about 

6 percent high density, so the same percentages were 

assumed to be true in 1977. Based on a review of aerial 

photographs in the Rainbow Springs basin, Jones and 

others (1996) determined that the approximate percentages of property covered by turf are 34 percent for high 

density, 66 percent for medium density, and 17 percent 

for low density residential. Jones and others (1996) also 

estimated that about 2 pounds (about 0.9 kg) per year of N 

was applied per 1,000 ft2

 (0.023 acres) of turfgrass. 

Applying the same method for residential land in the 

Silver Springs basin results in calculated N loads of about 

114,000 kg for 1977 and 727,000 kg for 1995 (fig. 11).

Estimates of N loads from livestock manure were 

calculated for 1940-2000 based on animal population 

data from the U.S. Department of Agriculture (1997; U.S. 

Department of Agriculture, unpublished data), including 

beef and dairy cattle, calves, hogs and pigs, sheep, horses, 

turkeys, and pullets and chickens. Nitrogen produced by 

each species was calculated based on a methodology 

described by the U.S. Geological Survey (1998) that 

calculates a load based on body weight and number of 

animals. As a result of various handling methods, the fraction of the total N load generated by livestock that reaches 

the ground water can vary widely. Estimates as low as 10 

to 30 percent are not unreasonable (Katz and others, 

1999); the relative contributions based on those estimates 

also are shown in figure 11.
20 Chemistry of Ground Water in the Silver Springs Basin, Florida, with an Emphasis on Nitrate

Another source of nitrogen is human wastes, either 

from municipal waste treatment systems or from OSDS, 

commonly called septic tanks. Estimates of the nitrogen 

output from such systems and the amount of nitrogen 

removed from the systems as nitrogen gas (N2) due to 

denitrification in the soil zone can vary widely; therefore, 

calculating the loads is complex. An attenuation by denitrification of 20 percent was estimated by Otis and others 

(1993); rates as high as 80 percent were reported by 

Horsley and others (1996).

Based on housing data from the U.S. Census Bureau 

(2002), approximately 36 percent of the dwelling units in 

Marion County use public sewers for waste disposal; the 

remaining 64 percent use OSDS. No additional data on 

the number or distribution of OSDS in the County were 

available. Also, there are no data from municipal wastewater treatment facilities prior to 1990 and N removal is 

not part of the treatment process. Therefore, the N load 

from all sources of wastewater was estimated on a per 

capita basis. The total population of Marion County was 

multiplied by a total N load of 4.09 kilograms per year 

(kg/yr) (Otis and others, 1993; Katz and others, 1999) 

(table 1). As with the load from fertilizers, it was not 

possible to determine the proportion of the wastewater 

load actually generated within the Silver Springs springshed. The loads from agricultural practices generally are 

decreasing (1975-2000), but loading from wastewater is 

increasing as the population increases (fig. 11).

Land Use in the 10-Year Contributing Area

Based on the conclusion by Faulkner (1973) that most 

of the flow to Silver Springs is relatively shallow (and, 

therefore, rapid), the area contributing recharge to the 

spring within a 10-year travel time (figs. 5 and 12) was 

delineated by using particle tracking and a computer model 

of ground-water flow in peninsular Florida (Sepúlveda, 

2002; N. Sepúlveda, U.S. Geological Survey, written 

commun., 2003). The methodology used to perform the 

particle-tracking is described by Shoemaker and others 

(2003). The presence of a smaller, discontinuous area to the 

east of the Ocklawaha River results because although the 

area along the Ocklawaha River is a discharge area for the 

Upper Floridan aquifer, recharge that enters the Upper 

Floridan aquifer east of the river (probably by upward 

movement from the Lower Floridan aquifer) does 

discharge from Silver Springs (Shoemaker and others, 

2003). The blocky (rather than smooth) outline of the 

contributing area (fig. 12) is the result of the coarseness of 

the finite-difference grid used in the model. Land use 

within the 10-year contributing area was examined (figs. 

10 and 12). An increase in urban areas is evident. Also, 

some land classified as agricultural in 1977 was classified 

as rangeland in 1995; some areas classified as forest in 

1977 were classified as recreation/open land in 1995.

Nitrogen loads from wastewater and for lawn fertilization were estimated for the 10-year contributing area 

because the area is mostly urban and these are the main 

sources of N. The wastewater load (regardless of whether 

disposed through a municipal system or by OSDS) was 

estimated based on the total population of the 10-year 

contributing area in 1990, calculated from census data 

(U.S. Census Bureau, 2002). The total population of that 

area (97,330) was multiplied by an estimated per capita N 

load of 4.09 kg/yr (Otis and others, 1993), resulting in a 

load estimate of about 398,000 kg/yr. The load from residential turf fertilization was estimated by using the 1995 

land-use coverage data and the same assumptions for 

application rates as described above from Jones and others 

(1996). The total annual load for lawn fertilization in the 

10-year contributing area was estimated to be 770 kg, 

indicating that wastewater potentially is a more significant source of nitrogen than fertilizer within the 10-year 

contributing area.

Chemistry of Ground and Spring Water 

During 2001-2002, water samples were collected 

from 56 wells in the study area and from three springs of 

the Silver Springs group. Data for field measurements, 

major ions, and nutrients are shown in appendix A. Wells 

were selected to be representative of the major land-use 

types in the study area based on 1995 land-use data. Land 

use at each sampled well was determined using GIS techniques and confirmed by field observations. Also, some 

wells were located in the area where the Upper Floridan 

aquifer is confined and others in areas where the aquifer is 

unconfined. Depth information was not available for all 

wells sampled; however, it is likely that all wells are 

representative of the Upper Floridan aquifer because for 

economic reasons, wells generally are completed in the 

shallowest productive unit.

Major Ions and Trace Constituents

Calcium, sulfate, and chloride are constituents that 

generally are of importance in ground water because they 

can affect the levels of treatment needed for water supply. 

In the Silver Springs basin, calcium concentrations in water 

from the 56 wells sampled during this study ranged from 24 

to 176 mg/L, with a median value of 71.5 mg/L (fig. 13).
Chemistry of Ground and Spring Water 21

Figure 12. 1977 and 1995 land use in the area contributing recharge to Silver Springs based on travel 

times up to 10 years.
22 Chemistry of Ground Water in the Silver Springs Basin, Florida, with an Emphasis on Nitrate

Sulfate values ranged from 0.6 to 400 mg/L, with a 

median of 20 mg/L. These are typical ranges for water 

withdrawn from the carbonate rocks of the Upper Floridan aquifer. Sulfate concentrations can range widely 

because of the presence or absence of sulfate minerals 

such as gypsum in the aquifer (Sacks, 1996). Sulfate 

concentrations generally increase with depth in the Upper 

Floridan aquifer (Faulkner, 1973). The maximum values 

of both calcium and sulfate occurred in the same well (9), 

which is one of the deepest wells sampled (210 ft). On the 

other hand, well 4 was 220 ft deep but had a sulfate 

concentration of only 13 mg/L and a calcium concentration of 24 mg/L (app. A). The highest chloride concentration measured was 230 mg/L in well 48, which also had a 

sodium concentration of 140 mg/L. This sample, 

however, may have been affected by backflow from a 

water softener. The next highest chloride concentration 

measured, 88 mg/L, was in water from a well (11) near a 

municipal wastewater land application facility (app. A).

The data collected during this study were compared 

to data from a previous study (Phelps, 1994) collected in 

1989-90 also using standard USGS protocols (Wood, 

1976). During that study, samples were collected from 25 

wells without consideration to land use at the site (Phelps, 

1994). Six wells (18, 29, 30, 33, 35, and 41) were sampled 

during both study periods. The median calcium value for 

the 1989-90 data was 82 mg/L, compared to a median of 

72 mg/L for the present study. There was a greater variation in sulfate concentrations during the present study. 

The median for the 1989-90 samples was 45 mg/L and the 

Figure 13. Ranges of calcium, sulfate, and nitrate-N concentrations for water samples collected in 1989-90 and 

during this study, 2001-02.
Chemistry of Ground and Spring Water 23

maximum value was about 220 mg/L; whereas, the 

median for the 2001-02 data was lower, only about 20 mg/L, 

but the maximum value was much greater, about 

400 mg/L (fig. 13). This difference in the maximum 

value probably is because the samples from the present 

study may have included more deep wells, and thus, 

represent a wider range of hydrogeologic conditions than 

the wells sampled during the previous study.

Calcium and sulfate concentrations have been 

measured by the USGS in water from the Main Spring of 

Silver Springs since 1956. Calcium concentrations range 

from 48 to 78 mg/L, and sulfate concentrations range from 

about 30 to 58 mg/L (fig. 14). Calcium concentration 

shows a slight increasing trend (determined by regression 

analysis), although the correlation is relatively weak 

(r2

=0.31). No trend is apparent in sulfate concentration.

The trace constituents boron (B) and manganese 

(Mn) were analyzed in ground-water samples because 

both are contained in synthetic fertilizers and B also is 

found in some laundry products. Igneous and metamorphic rocks are important natural sources of these elements 

(Hem, 1985). In Florida, however, such rocks are present 

only at great depth below the aquifer system. Therefore, 

the presence of B and Mn in ground water probably indicates the effects of either fertilizer use or wastewater. 

Concentrations of B ranged from 9 to 74 micrograms per 

liter (μg/L), with the highest concentration occurring in 

water from a monitor well located in a wastewater 

sprayfield (well 11, fig. 5 and app. A). The next highest 

concentration (51 μg/L) was in water from well 8 in an 

undeveloped area of a park, which was not likely to have 

been impacted by either wastewater or current fertilizer 

use. However, the site is shown as a citrus grove on the 

USGS 7.5-minute Lake Weir topographic map (dated 

1970 and photorevised in 1980); therefore, the high B 

concentration at well 8 may result from historic fertilizer 

applications. Concentrations of Mn in ground-water 

samples ranged from less than the detection limit of 1.0 to 

530 μg/L. Water from a well in an urban commercial area 

(well 35, fig. 5 and app. A) had the highest concentration 

of Mn. This may be the result of fertilizer applications.

Figure 14. Calcium and sulfate concentrations in water from the Silver 

Springs Main Spring, 1955-2003.
24 Chemistry of Ground Water in the Silver Springs Basin, Florida, with an Emphasis on Nitrate

A consistent relation does not seem to exist between 

the concentrations of B and Mn in ground-water samples. 

For example, at well 8, where the B concentration was 

51 μg/L (the second highest value measured), the Mn 

concentration was 16 μg/L (four other wells had higher 

Mn concentrations). Water from well 35, which had the 

highest Mn concentration, 530 μg/L, had a B concentration of only 20 μg/L. Likewise, there was no consistent 

relation between B and Mn concentrations and land use. 

For example, land use at well 44 is low-density residential 

and concentrations of B and Mn in the water are 40 and 

60 μg/L, respectively; whereas the concentrations of both 

constituents were significantly lower in water from other 

wells (wells 4, 24, and 50; fig. 5 and app. A) with similar 

land use. Differences in past fertilizer usage, as well as 

the presence or absence of an OSDS, could account for 

the apparent lack of a relation to land use.

Dissolved Oxygen and Nutrients

Dissolved oxygen (DO) measured in the field can be 

an indicator of recently recharged ground water because 

there generally are no sources of oxygen in the aquifer 

and the longer the water is contained in an aquifer, the 

lower the DO becomes (Katz and others, 1999). DO 

concentrations ranged from 0.1 mg/L at well 38 to 8.3 

mg/L at well 5 (fig. 15). The water from well 38 (fig. 5) 

had a slight hydrogen sulfide odor, indicating that it probably came from relatively deep in the aquifer. The well is 

in an area where the Upper Floridan aquifer is confined 

(fig. 15), so the low DO is expected. DO generally is high 

in unconfined areas of the Upper Floridan aquifer, but 

high values also occur in confined areas, such as in the 

southern part of the basin. This could be due to local variations in aquifer confinement that are not apparent on a 

regional scale. In general, areas with high levels of DO in 

the ground water could be more vulnerable to rapid infiltration of N loads from the land surface.

Dissolved oxygen in water discharging from the Main 

Spring of Silver Springs ranged from 1.3 to 2.8 mg/L 

during the study and the concentration was slightly higher 

in water from the Abyss and the Blue Grotto (app. A). 

A total of 66 DO measurements by the USGS in the Main 

Spring from 1977 to 2002 ranged from 1.3 to 6.8 mg/L, 

with a median of 2.3 mg/L. In general, DO from the Main 

Spring increases with increasing discharge.

Nitrogen occurs in water in several forms: NO2

-

, 

NO3

-

, and ammonium (NH4

+

) (Hem, 1985). Ammonium 

ions are strongly adsorbed on mineral surfaces and nitrite 

is unstable in aerated water. Nitrate is mobile in water 

and is stable in ground water under some conditions. 

Laboratory analyses usually report a combined value for 

NO2

-

-plus-NO3

-

 as N. Values for NO2

-

 are determined as 

well (and often are less than the detection limit of 0.02 

mg/L) so the concentration of NO3

-

 (nitrate-N) is approximately equal to the concentration of NO2

-

-plus-NO3

-

. 

Values of nitrate-N for the study area ranged from 

less than the detection limit of 0.02 to 12 mg/L as N, with 

a median value of 1.2 (fig. 13). The areal distribution of 

nitrate-N (fig. 16) shows that, similar to DO, concentrations vary and do not seem to be related to the degree of 

aquifer confinement as shown by the regional delineation 

of geologic units. Also, although water samples with 

higher nitrate-N concentrations generally had higher DO 

values, there does not seem to be a predictive relation 

between the two constituents (fig. 17). The grouping of 

nitrate values in figure 16 is based on ranges described by 

Madison and Brunett (1985) as follows:

Less than 0.2 mg/L Assumed to represent background 

conditions.

0.2-3.0 mg/L

Transitional; concentrations may or may 

not represent influence from human 

activities.

3.01-10 mg/L May indicate elevated concentrations 

resulting from human activities.

More than 10 mg/L

Exceeds Maximum Contaminant Level 

(MCL) for nitrate-N (U.S. Environmental 

Protection Agency, 2003).

One well sampled (well 49) exceeded the MCL for nitrate-N.

Concentrations of phosphorus (P), another nutrient, 

were analyzed as total phosphorus and total orthophosphate. Phosphorus is not as mobile as nitrogen and 

because of the ability of phosphate to sorb onto metal 

oxides and carbonate minerals, concentrations in natural 

water greater than a few hundredths or tenths of a milligram per lieter are rare (Hem, 1985). Concentrations of 

total orthophosphate in water from the 56 wells sampled 

ranged from 0.01 to 0.89 mg/L, with a median value of 

0.03 mg/L; total P ranged from 0.02 to 2.7 mg/L with a 

median of 0.04 mg/L (fig. 18). Hawthorn Group sediments can contain phosphate-bearing minerals such as 

apatite, but because those minerals have a low solubility 

and the Hawthorn sediments are relatively thin and 

discontinuous in the study area, they probably are not a 

major source of phosphate in ground water. Hem (1985) 

reported that P is present in animal metabolic waste and 

that domestic and industrial sewage effluents probably are 

sources of P in surface water. In the study area, where 

wastewater is disposed by land application instead of to 

surface water, effluent could thus be a source of P to the 

ground water.
Chemistry of Ground and Spring Water 25

Figure 15. Dissolved oxygen concentrations in water from the Upper Floridan aquifer, Silver Springs basin.
26 Chemistry of Ground Water in the Silver Springs Basin, Florida, with an Emphasis on Nitrate

Figure 16. Nitrate-N concentrations in water from the Upper Floridan aquifer, Silver Springs basin.
Chemistry of Ground and Spring Water 27

Figure 17. Nitrate-N as a function of dissolved 

oxygen in ground-water samples.

Figure 18. Total phosphorus and total 

orthophosphate in ground water, 1989-90 

and 2000-01.
28 Chemistry of Ground Water in the Silver Springs Basin, Florida, with an Emphasis on Nitrate

Nutrient concentrations for water discharging from 

the Main Spring of Silver Springs for the period of record 

(1956-2004) also were compiled. The concentration of 

nitrate-N in water discharging from Silver Springs generally has increased over time (fig. 2). During the study 

period (1999-2002) nitrate-N concentrations in water 

from the Main Spring were approximately 0.9 mg/L 

(ranging from 0.8 to1.1 mg/L). Phosphorus data have 

been collected for water samples from the Main Spring 

since 1967. Total P values ranged from less than the 

detection limit of 0.02 to 0.07 mg/L; total ortho P ranged 

from 0.02 to 0.07 mg/L. Neither phosphorus constituent 

shows an increasing temporal trend.

Comparison to Data from 1989-90

Nutrient data from 56 wells sampled during this 

study also were compared to data from 25 wells sampled 

during 1989-90 (Phelps, 1994). The median nitrate-N 

concentration for water samples from the previous study 

was 1.04 mg/L compared to a median of 1.2 mg/L for the 

present study (fig. 13). The maximum value from the 

previous study was 3.6 mg/L whereas the maximum 

during this study was 12 mg/L. Total orthophosphate had 

a similar range in both studies (fig. 18), although the 

median for the 1989-90 data was higher (0.06 mg/L) than 

for the data from the present study (0.03 mg/L). The 

maximum total P value for the 1989-90 data was 0.22 

mg/L, compared to a maximum of 2.7 mg/L during the 

present study. Except for the maximum value, total P 

values during the present study were in the same range as 

those from the previous study (1989-90). Median total P 

for 1989-90 data was 0.05 mg/L compared to 0.04 mg/L for 

the present study.

Six wells were sampled during both studies; three of 

those wells are part of the Florida Department of Environmental Protection Very Intensely Studied Area network 

(2003). There are insufficient data to determine trends in 

nitrate-N concentrations (table 2). In water from one well 

(41), concentrations apparently increased; in another well 

(30), there was an apparent decrease. In the other wells, 

concentrations show little change.

Relation of Nitrogen Concentrations to Land Use

A question of interest to scientists and water managers is this: What are the effects of land use on groundwater chemistry, especially in a karst area such as the 

Silver Springs ground-water basin? To obtain a better 

understanding of the relation between land use and 

ground-water chemistry, wells were grouped according to 

reported 1995 land use at the site and concentrations of 

nitrate-N in water samples were compared (table 3). 

Median concentrations were calculated for each group. 

The median was calculated for the 30 wells located in all 

types of urban areas; medians also were calculated for 

three urban subgroups of low-density residential, 

medium- and high-density residential, and for the group 

consisting of commercial, industrial, and recreation uses. 

The commercial-industrial-recreational group had the 

highest variability in concentration. The number of wells 

in some land-use type areas, such as transportation and 

utilities, is too small to provide a statistical characterization of that category. Also, 1995 land use may not accurately represent historic land use, which could have 

significantly affected ground-water chemistry.

The agricultural land-use areas had the highest median 

nitrate-N concentration (1.70 mg/L). The highest value of 

nitrate-N in the study (12 mg/L) also was in an area of agricultural land use, although it is uncertain whether the high 

concentration is related to land-use activities or to proximity of a septic tank on the property. For this reason, the 

statistics for agricultural land use also were calculated for 

all samples excluding the sample with the highest concentration. The median for that group of samples was 1.55 

mg/L. The median nitrate-N for all urban areas was 1.15 

mg/L. Low-density residential had the highest median of 

the urban subgroups, 1.40 mg/L. Because fewer wells are 

located in rangeland and forests, those land-use areas were 

grouped together. That group had the lowest median 

concentration, 0.65 mg/L. The relatively high value (1.57 

mg/L) for the transportation/utilities land use could reflect 

the effects of stormwater runoff for road rights-of-way or 

could be affected by nearby land-use activities; for example, well 12 is a monitoring well downgradient from a 

wastewater land application site, but it is located in an electrical power transmission right-of-way.

The nonparametric Wilcoxon rank-sum test (Insightful 

Corporation, 2001) was used to test for differences in 

median nitrate concentrations in ground-water samples 

from agricultural, urban, and rangeland and forest (combined) land-use areas. Comparisons were made between 

each set of samples. At the 0.05 significance level, the 

differences between all data sets were significant.

 Wells in the area contributing recharge to Silver 

Springs within an estimated 10-year travel time also were 

grouped by land-use type (table 3). The maximum nitrateN concentration for all urban land-use types occurred 

within the 10-year contributing area; the medians for all 

urban data and urban data within the 10-year contributing 

area were nearly the same. For agricultural land-use areas, 

the median values (both with and without the 12 mg/L 

value) were slightly lower in the 10-year contributing area 

than in the entire study area. For rangeland and forest, the
Chemistry of Ground and Spring Water 29

Table 2. Comparison of historic and current nitrate-N concentrations in filtered 

ground-water samples.

Well 

No. Site Date

Nitrate-N

(milligrams 

per liter)

1995 land use

18 290838082103501 6/27/90 2.5 Specialty farms (horse farms)

5/16/01 2.7

29 291123082065001 5/23/90a

a

Data from Florida Department of Environmental Protection Very Intensely Studied Area database.

.52b

b

Unfiltered sample.

Medium-high density residential

1/26/95 .33

8/14/01 .28

30 291123082075402 5/23/90a .84b Commercial-parks or zoos

1/18/95 .72

8/15/01 .38

33 291148082072702 5/24/90a .60b Medium-high density residential

1/25/95 1.2

8/15/01 .84

35 291226082042001 6/21/89 1.20b Commercial-retail sales

5/10/01 1.10

41 291320082042301 6/28/90 .15 Commercial-light industrial

5/08/01 .42

Table 3. Concentrations nitrate-N in ground-water samples grouped by 1995 land use at well locations.

[LUCODE refers to appendix B; concentrations in milligrams per liter; <, less than; --, no data. Number of wells: All, all study wells; 10-year, subset 

of study wells sampled within 10-year contributing area]

LUCODE 1995 land use

No. 

of wells Minimum Maximum Median

All 10-year All 10-year All 10-year All 10-year

 <2000 All urban 29 20 <0.02 <0.02 5.9 5.9 1.15 1.10

 <1200 Low-density residential 10 4 <.02 .65 2.2 2.2 1.40 1.70

 1200-<1400 Medium- and high-

 density residential

7 6 <.02 <.02 3.1 1.8 1.10 1.05

 1400-1999 Commercial, industrial,

 recreation

12 10 .37 .37 5.9 5.9 1.15 .88

2000-2999 Agricultural 13 5 .05 .85 12 12.0 1.70 1.20

2000-2999 Agricultural (without the 

maximum value of 12)

12 4 .05 .85 9.9 1.7 1.55 1.15

3000-4999 Rangeland and forest 11 4 <.02 <.02 2.2 2.2 .09 .65

8000-9000 Transportation, utilities 3 1 .88 -- 4.0 -- 1.57 .91
30 Chemistry of Ground Water in the Silver Springs Basin, Florida, with an Emphasis on Nitrate

median value in the 10-year contributing area is higher 

than the entire study area, perhaps because there are 

urbanized areas upgradient of some of the wells in rangeland and forest areas within the 10-year contributing area.

These results are somewhat different from those 

reported by the USGS National Water Quality Assessment (NAWQA) Program. In that study, water from wells 

in unconfined areas of the Upper Floridan aquifer in 

northeastern Florida and eastern Georgia had nitrate-N 

concentrations ranging from less than 0.02 to 10.00 mg/L, 

with a median of 0.28 mg/L, in urban areas. In agricultural areas (Berndt, 1996), the range was from less than 

0.02 to 2.5 mg/L, with a median of 0.40 mg/L. These 

differences may be due in part to the greater number of 

wells sampled (42 in agricultural areas and 96 in urban 

areas), or because the range of geologic conditions varied 

more widely over the NAWQA study area. Also, the 

specific land-use practices (that is, type of crops raised) 

may have been different in the NAWQA area than in 

Marion County.

Nitrogen Isotopes

To provide insight about the sources of nitrogen in 

ground water in the study area, water samples were 

collected for analysis of the ratio of 15N/14N at 37 wells 

and three springs of the Silver Springs group. Wells were 

selected to be representative of the various land-use types 

in the study area. Values in ground-water samples ranged 

from -0.5 to 11.5 per mil (fig. 19). In general, values less 

than 6 per mil are indicative of the effects of artificial 

(inorganic) nitrogen (fertilizers). Values between 6 to 9 

per mil are representative of mixed inorganic and organic 

sources or a soil organic source; values greater than 9 are 

indicative of organic nitrogen (from human or animal 

wastes) (Coplen, 1993; Katz and others, 1999). The 

median value for all ground-water samples, 4.9 per mil, is 

within (but near the top of) the range for inorganic 

sources.

 The areal distribution of 15N/14N is shown in figure 20.

The values of 15N/14N, along with the land use and geologic 

formation at each well, are shown in appendix A. Well sites 

were grouped by 1995 land-use type so statistics for 

15N/14N values could be computed for each group (fig. 19). 

For 17 wells in urban land-use areas, the ratio ranged from 

-0.5 to 10.8 per mil, with a median of 5.4 per mil. Values for 

agricultural land-use areas (11 wells) ranged from 1.9 to 8.9 

per mil, with a median of 4.8 per mil. For rangeland and 

forested land areas, the range was 2.2 to 11.5 per mil, with 

a median of 4.1 per mil. The three sites in transportation, 

communication, or utilities land-use areas had values of 

-0.2, 2.7, and 5.2 per mil, respectively. The median for 

urban land uses (5.4 per mil) is slightly higher (indicating 

more organic nitrogen) but the medians for all groups were 

within the range of generally inorganic sources. Land-use 

designations should be used with caution, however. For 

example, land use at well 11 is designated as agricultural 

although it is located within a spray field for wastewater 

land application. The 15N/14N ratio for that sample was 8.9 

per mil, which is close to the range indicating mostly 

organic N. A nearby monitoring well (well 12) is in the 

right-of-way for an electrical power transmission line and 

land use for that site is indicated as such. The 15N/14N ratio 

for that sample was 5.2 per mil, which is in the mixed 

sources range. The 15N/14N ratio for the sample with the 

highest nitrate-N concentration (12 mg/L, well 49) was 7.1 

per mil. The two wells with very low ratios (well 17, 

nitrate-N = 0.94 mg/L, and well 27, nitrate-N = 0.37 mg/L) 

are not known to be near sources of fertilizer or wastewater 

N. Although the Upper Floridan aquifer is semiconfined at 

these well locations, the 15N/14N ratios of the water could be 

representative of nitrate-N from atmospheric deposition or 

could represent effects of upgradient fertilizer use.

The Main Spring of Silver Springs was sampled four 

times during the study for 15N/14N ratios (table 4). Two 

distinct nitrogen isotope ratios were observed: two values 

indicating inorganic nitrogen sources (3.7 and 3.8 per mil) 

and two values indicating mixed sources with a strong

MAXIMUM 11.5

MEDIAN 4.9

MINIMUM -0.5

ORGANIC NITROGEN

MIXED NITROGEN

INORGANIC NITROGEN

15N / 14N, IN PER MIL

12.0

10.0

8.0

6.0

4.0

2.0

0

-2.0

LAND USE MEDIAN 15N / 14N

URBAN

AGRICULTURE

RANGELAND

AND FOREST

5.4 PER MIL

4.8 PER MIL

4.1 PER MIL

Figure 19. Range of 15N/14N for ground-water samples.
Chemistry of Ground and Spring Water 31

Figure 20. Areal distribution of 15N/14N for ground-water samples.
32 Chemistry of Ground Water in the Silver Springs Basin, Florida, with an Emphasis on Nitrate

influence of organic sources (8.1 and 8.7 per mil). It has 

been hypothesized that during low flows, the discharge of 

Silver Springs is derived mainly from flow through the 

porous aquifer matrix and, therefore, might be more influenced by the load of inorganic nitrogen that was added 

continuously to the basin over time by fertilizer applications. All of the nitrogen isotope determinations were 

made at spring discharge values less than the long-term 

average flow; for this range of flow there was no apparent 

relation between 15N/14N ratio and spring discharge. 

Divers reported at least two conduits discharging water of 

different temperatures in the cavern beneath the Main 

Spring (R. Spechler, U.S. Geological Survey, Orlando, 

Fla., oral commun., 2003). Thus, it seems possible that 

the 15N/14N ratios measured during this study may reflect 

differences in the placement of the sampling pump during 

the various sampling events, rather than an actual variation of the ratio. Another possible explanation is that the 

distribution of antecedent rainfall in the recharge areas 

could cause one or the other conduit system to be contributing more flow during different sampling events. Variations in tritium concentrations in water from the Main 

Spring (Faulkner, 1973), discussed in a later section of 

this report, also point to a rather rapidly changing distribution of source waters to the Main Spring.

To summarize nitrate-N and nitrogen isotope results, 

nitrate-N concentrations in agricultural areas were higher 

(median value 1.7 mg/L) than in urban (median 1.15 

mg/L) or rangeland and forest areas (median 0.09 mg/L). 

In agricultural areas, the median 15N/14N was 4.8 per mil, 

indicating mostly inorganic (fertilizer) sources. In urban 

areas, the median 15N/14N was 5.4 per mil, indicating 

more influence of organic N sources. Thus, in both agricultural and urban areas, fertilizer is an important inorganic source of N in ground water, and therefore, in spring 

water as well. The influence of organic N is more apparent 

in urban areas than in agricultural areas.

Wastewater Indicators

Because nitrogen isotope data indicated mixed organic 

and inorganic nitrogen sources for many water samples, 

water samples also were analyzed for organic constituents 

commonly found in domestic and industrial wastewater to 

gain additional insight into the sources of nitrogen in 

ground water and in Silver Springs. Thirty-five wells and 

three springs of the Silver Springs group were sampled for 

a suite of 63 compounds (table 5). Compounds found in 

detergents and personal care products, pesticides, and 

compounds that result from industrial processes were 

included. Some of the compounds are known or suspected 

endocrine disrupters. A total of 38 compounds was 

detected, nearly all in very low concentrations.

 The most frequently detected compound was the 

insecticide N,N-diethyl-meta-toluamide (DEET), which 

was detected in water from 27 of the 35 wells sampled 

(fig. 21). DEET is an insecticide designed for application 

to the skin to repel, rather than kill, mosquitoes. The U.S. 

Environmental Protection Agency (1998) determined that 

DEET is in Toxicity Category III (slightly toxic, the 

second lowest of four categories). The chemical is of low 

solubility and does not break down easily. DEET is not

Table 4. Isotopic data for water samples from Silver Springs, 2001-2002.

[Isotopic ratios are in per mil. 2H/1H, deuterium, hydrogen; 15N/14N, nitrogen-15, nitrogen-14; 18O/16O, oxygen-18, oxygen-16; mg/L, milligrams per liter; μS/cm, 

microseimens per centimeter; o

C, degrees Celsius; ft3/s, cubic foot per second; dup., duplicate; --, not analyzed]

Site Site name Date

Dissolved 

oxygen 

(mg/L)

pH

(units)

Specific 

conductance

(μS/cm)

Temperature

(

o

C)

2

H/1H 15N/14N 18O/16O Tritium, 

units

Discharge, 

(ft3

/s)

02239500 Main 5/8/2001 1.3 6.7 451 23.2 -14.3 8.7 -3.05 3.4 395

02239500 (dup.) Main 5/8/2001 1.3 6.7 451 23.2 -13.9 8.5 -3.0 3.3 395

291258082030900 Main 11/15/2001 2.4 7.2 471 23.2 -- 3.8 -- -- 577

291258082030900 Main 1/29/2002 1.5 7.4 458 23 -15.4 8.1 -3.08 3.4 496

291258082030900 Main 7/18/2002 2.7 6.4 466 23.3 -- 3.7 -- -- 412

291253082030600 Abyss 11/15/2001 3.7 7.2 468 23.6 -- 4.9 -- -- --

291253082030600 Abyss 1/29/2002 3.7 7.3 492 23.7 -18.1 -- -3.4 2.7 --

291255082030000 Blue Grotto 11/15/2001 3.2 7.3 443 24.5 -- 5.8 -- -- --

291255082030000 Blue Grotto 1/29/2002 4 7.4 466 23.7 -18.3 -- -3.41 2.7 --
Chemistry of Ground and Spring Water 33

Table 5. Summary of compounds commonly detected in wastewater and detections in water samples. 

[Endocrine Disrupting Potential: K, known, S, suspected. F, fungicide; H, herbicide; I, insecticide; GUP, general use pesticide; FR, flame retardant; 

Manuf, manufacturing; --, not detected] 

Compound names

Endocrine 

distrupting 

potential

Chemical 

Abstract Service

registry Nos. 

Common use, application, 

or occurrence

No. 

of 

detections

1,4-Dichlorobenzene S 106-46-7 moth repellent, fumigant, deodorant --

1-Methylnaphthalene 90-12-0 nearly equal concentrations (2-5%) in gasoline/diesel/crude 1

2,6-Dimethylnaphthalene 58-14-2 indicator of diesel, kerosene (not much in gasoline) 1

2-Methylnaphthalene 91-57-6 nearly equal concentrations (2-5%) in gasoline/diesel/crude 2

3-beta-Coprostanol 360-68-9 usually a carnivore fecal indicator 1

3-Methyl-1(H)-indole (skatol) 83-34-1 fragrance: odor in feces and coal tar 1

3-tert-Butyl-4-hydroxy anisole (BHA) K 25013-16-5 antioxidant, preservative --

4-Cumylphenol K 599-64-4 nonionic detergent metabolite --

4-n-Octylphenol K 1806-26-4 nonionic detergent metabolite --

4-tert-Octylphenol K 140-66-9 nonionic detergent metabolite --

5-Methyl-1H-benzotriazole 136-85-6 antioxidant in antifreeze, deicers --

Acetophenone 98-86-2 fragrance: soap, detergent, tobacco; flavor: beverages 1

Acetyl hexamethyl tetrahydronaphthalene 

(AHTN)

 21145-77-7 fragrance: musk, widespread usage, persistent in 

ground water

1

Anthracene 120-12-7 wood preservative, in tar/diesel/crude (not gasoline) --

Anthraquinone 84-65-1 Manuf dye/textiles; seed treatment, bird repellent --

Benzo(a)pyrene K 50-32-8 regulated polychlorinated aromatic hydrocarbon, used in 

cancer research

--

Benzophenone S 119-61-9 fixative for perfumes and soaps 2

beta-Sitosterol 83-46-5 generally a plant sterol 1

beta-Stigmastanol 19466-47-8 generally a plant sterol --

Bisphenol A K 80-05-7 Manuf polycarbonate resins; antioxidant, FR 11

Bromacil 314-40-9 H, GUP; greater than 80% noncrop grass/brush control --

Bromoform 75-25-2 byproduct of wastewater ozonation, military 

uses/explosives

1

Caffeine 58-08-2 medical: diuretic; highly mobile/biodegradable 3

Camphor 76-22-2 flavor, odorant, in ointments 1

Carbaryl K 63-25-2 I, crop and garden uses, low environmental persistence --

Carbazole 86-74-8 Manuf dyes, explosives, and lubricants, I --

Chlorpyrifos K 2921-88-2 domestic pest/termite control; highly restricted (2000) --

Cholesterol 57-88-5 often a fecal indicator, also a plant sterol 4

Cotinine 486-56-6 primary nicotine metabolite --

Diazinon K 333-41-5 I, > 40% nonagricultural uses, ants, flies, etc. --

Dichlorvos S 62-73-7 I, pet collars, fly spray; breakdown of naled & trichlofon --

d-Limonene 5989-27-5 F, antimicrobial, antiviral; fragrance in aerosols 1

Fluoranthene 206-44-0 common in coal tar/asphalt (not gasoline/diesel) --

Hexahydrohexamethyl 

Cyclopentabenzopyran (HHCB)

 1222-05-5 fragrance: musk; widespread usage, persistent in 

ground-water

--

Indole 120-72-9 pesticide inert, fragrance: coffee 1

Isoborneol 124-76-5 fragrance: perfumery, disinfectants --

Isophorone 78-59-1 solvent for lacquers, plastics, oils, silicon, resins 4

Isopropylbenzene (cumene) 98-82-8 Manuf phenol/acetone; component of fuels/paint thinner --

Isoquinoline 119-65-3 flavors and fragrances --

Menthol 89-78-1 cigarettes, cough drops, liniment, mouthwash --
34 Chemistry of Ground Water in the Silver Springs Basin, Florida, with an Emphasis on Nitrate

sprayed over large areas for insect control. The primary 

use is as a personal insect repellent for humans, but DEET 

also has been used on some insect repellent collars for pets 

and as a spray to repel flies from horses. DEET enters the 

wastewater when the user bathes or launders clothing. It 

was beyond the scope of this study to estimate the relative 

importance of use on animals as a source of DEET (as 

compared to human use) in the Silver Springs basin; 

however, based on widespread detections in all land-use 

areas, wastewater is likely the major source of DEET in 

the study area. DEET appears to be a useful tracer for the 

presence of wastewater. In a nation-wide sampling of 

surface water (139 streams), Buxton and Kolpin (2002) 

detected DEET in 74 percent of the samples.

The presence or absence of DEET in ground-water 

samples was not related to land use. The compound was 

detected in water from 11 of 15 wells located in all urban 

land use areas (8 of 10 residential areas and 3 of 5 

commercial, industrial or recreational areas). DEET was 

detected in water from 8 of 9 wells in agricultural areas, 

5 of 8 wells in rangeland or forested areas, and all 3 wells 

in transportation or power transmission areas.

 The presence or absence of DEET in ground water 

was related to geologic conditions at well sites, however. 

Of the 35 wells sampled, nine were in the outcrop area of 

the Ocala Limestone, the principal water-bearing unit of 

the Upper Floridan aquifer, and DEET was detected in all 

nine samples. In contrast, all of the water samples in 

which DEET was not detected were collected from wells 

in areas where the Ocala Limestone is covered by younger 

sediments (fig. 21). The presence of DEET in some of the 

water samples from areas where the Upper Floridan aquifer is covered by younger sediments can be explained by 

recharge water containing DEET that enters the Upper 

Floridan aquifer in unconfined areas and moves downgradient to confined areas.

Metalaxyl 57837-19-1 H, F, GUP, soil pathogens, mildew, blight, golf turf --

Methyl salicylate 119-36-8 liniment, food, beverage, UV-adsorbing lotions --

Metolachlor 51218-45-2 H, GUP, indicator of agricultural drainage --

N,N'-diethyl-methyl-toluamide (DEET) 134-62-3 I, urban uses, mosquito control 30

Naphthalene 91-20-3 fumigant, moth repellent, about 10% of gasoline 1

Nonylphenol, diethoxy- (total) K 26027-38-3 nonionic detergent metabolite 2

Octylphenol, diethoxy- K 26636-32-8 nonionic detergent metabolite 1

Octylphenol, monoethoxy- K 26636-32-8 nonionic detergent metabolite --

para-Cresol S 106-44-5 wood preservative 3

para-Nonylphenol (total) K 84852-15-3 nonionic detergent metabolite 4

Pentachlorophenol S 87-86-5 H, F, wood preservative, termite control --

Phenanthrene 85-01-8 Manuf explosives; in tar/diesel/crude (not gasoline) --

Phenol 108-95-2 disinfectant, manuf of several products, leachate 27a

Prometon 1610-18-0 H, only noncrop areas, applied prior to blacktop --

Pyrene 129-00-0 common in coal tar/asphalt (not gasoline/diesel) --

Tetrachloroethylene 127-18-4 solvent, degreaser; veterinary: anthelminic 3

tri(2-Chloroethyl) phosphate S 115-96-8 plasticizer, FR 4

tri(Dichlorisopropyl) phosphate S 13674-87-8 FR --

Tributylphosphate 126-73-8 antifoaming agent, FR 2

Triclosan S 3380-34-5 disinfectant, antimicrobial (concern: induced resistance) 6

Triethyl citrate (ethyl citrate) 77-93-0 cosmetics, pharmaceuticals, widely used --

Triphenyl phosphate 115-86-6 plasticizer, resins, waxes, finishes, roofing paper, FR --

tris(2-Butoxyethyl) phosphate 78-51-3 FR 1

a

Phenol detections are observed in laboratory blanks, so no interpretation of phenol data is made.

Table 5. Summary of compounds commonly detected in wastewater and detections in water samples. (Continued)

[Endocrine Disrupting Potential: K, known, S, suspected. F, fungicide; H, herbicide; I, insecticide; GUP, general use pesticide; FR, flame retardant; 

Manuf, manufacturing; --, not detected] 

Compound names

Endocrine 

distrupting 

potential

Chemical 

Abstract Service

registry Nos. 

Common use, application, 

or occurrence

No. 

of 

detections
Chemistry of Ground and Spring Water 35

Figure 21. Wells where the insecticide DEET was detected in ground-water samples.
36 Chemistry of Ground Water in the Silver Springs Basin, Florida, with an Emphasis on Nitrate

Other compounds detected in ground-water samples 

included phenol (24 wells), bisphenol-A (10 wells), and 

triclosan (6 wells) (table 5). Some of the lower phenol 

occurrences may be erroneous because problems with 

phenol detections in laboratory blanks (S. Zaugg, USGS 

National Water Quality Laboratory, Denver, Colo., oral 

commun., 2002) make interpretation of low-level phenol 

data difficult. However, levels of the compound higher 

than 0.5 μg/L were quantified in samples from 16 of the 

24 wells.

In most samples, only one or two compounds were 

detected; however, samples from two wells (23 and 30) 

contained more compounds and in higher levels than 

other ground-water samples. Water from well 30, located 

in a park in downtown Ocala, contained DEET (5.8 μg/L) 

and 19 other compounds including caffeine (0.14 μg/L). 

Caffeine breaks down quickly (Heberer, 2002) and is 

considered an indicator of relatively recent recharge by 

wastewater. The concentration of cholesterol, a fecal indicator, was 5.2 μg/L. Phenol was detected at a concentration of 6.3 μg/L and the concentration of bisphenol-A was 

4.4 μg/L. The sample from well 23, in a recreation area 

that formerly was used as a pasture and is surrounded by 

residential areas using OSDS, contained three fecal indicators: 3-beta-coprostanol (2.4 μg/L); cholesterol (7.2 

μg/L); and 3-methyl-1(H)-indole (0.23 μg/L). Nonylphenol, diethoxy- (total) (NPEO2), a nonionic detergent 

metabolite and a known endocrine disrupter, and DEET 

also were detected in this sample.

Water samples from the Main Spring and two other 

springs of the Silver Springs group were analyzed for 

wastewater constituents in January 2002. To confirm 

results, the Main Spring was resampled in duplicate in 

July 2002. DEET was detected in all three spring samples 

from January and confirmed in the Main Spring in July. 

Levels ranged from the detection limit of approximately 

0.03 to 1.2 μg/L (app. A). Phenol was detected in two of 

the January samples and isophorone in one sample. 

Phenol also was detected in the July samples (0.34 and 

0.88 μg/L levels); caffeine was detected in one sample but 

not in the duplicate.

Dating Spring Water using Anthropogenic Tracers

In January 2002, water samples were collected from 

the Main Spring, the Abyss, and the Blue Grotto of the 

Silver Springs group and analyzed for concentrations of 

dissolved gases, CFCs, SF6, and 3

H/3Hetrit for the purpose 

of dating the young fraction of ground water discharging 

from the springs.

Spring-water samples were analyzed for dissolved 

gases (table 6) to determine if excess N2 (above equilibrium concentrations) or excess air were present. Excess 

air is trapped in pores of the unsaturated zone and 

dissolved under hydrostatic pressure. The amount of 

excess air present can be determined by comparing the 

ratio of N2 to argon (Ar) in air to the equilibrium solubility 

ratio of N2 to Ar in water (Busenberg and others, 1993).

The presence of excess N2 in water samples may indicate 

that denitrification is occurring or has occurred sometime 

in the past. Denitrification can reduce nitrate concentration and increase the 15N/14N values in an aquifer because 

the lighter isotope is used preferentially (Hubner, 1986; 

Mariotti and others, 1988). Concentrations of N2 in 

spring-water samples were consistent with atmospheric 

equilibrium during ground-water recharge (table 6; 

fig. 22), excess air values were low, and estimated 

recharge temperatures (about 23o C) were reasonable for 

Florida spring-water samples. These data (table 6; fig. 22), 

coupled with the fact that DO concentrations of the 

spring-water samples ranged from about 1.3 to 3.7 mg/L 

(app. A) (and denitrification is an anaerobic process), 

indicate that little, if any, denitrification is occurring in the 

immediate vicinity of the springs.

Anthropogenic activities released low but measurable quantities of 3

H, CFCs, and SF6 into the atmosphere 

during the later half of the twentieth century (Cook and 

Böhlke, 2000; Plummer and others, 1993). Rainfall incorporates the concentrations of the chemicals at the time it 

infiltrates into the ground. The ratio 3

H/3Hetrit and concentrations of SF6 and CFCs can be used to date relatively 

young (less than about 50 years) ground water. Gas 

exchange between air and the unsaturated zone is assumed 

to be rapid, but also that shallow ground water remains 

closed to gas exchange after recharge (Schlosser and 

others, 1989; Plummer and Busenberg, 2000; Busenberg 

and Plummer, 2000). The ratio 3

H/3Hetrit generally is not 

affected by contamination, sorption, or microbial degradation processes that can change the concentrations of SF6

or CFCs in various ways (Plummer and others, 1998; 

Busenberg and Plummer, 2000). The concentrations of 

3

H, 3Hetrit, CFCs, and SF6 can be affected by hydrodynamic dispersion and mixing of different age waters to 

varying extents (Solomon and Sudicky, 1991; Reilly and 

others, 1994). Also, some rocks are terrigenic sources of 

4

He and SF6, which can complicate the interpretation. For 

these reasons, several methods usually are used to date 

ground water; concordant ages from all methods indicate 

that the water was not affected by mixing of water from 

various sources or by dispersion.
Chemistry of Ground and Spring Water 37

Apparent ages of ground water are based on the 

piston-flow model assumption that after a tracer is 

isolated from the atmosphere at the time of recharge, it is 

incorporated into a parcel of ground 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 (which is not the case in an aquifer with 

dual porosity, such as the Floridan aquifer system). It is 

furthermore assumed that dispersion, diffusion, or advection of the tracer are negligible. The tracer concentration 

(or ratio, as is the case for 3

H/3Hetrit) is assumed to depend 

solely on a time-dependent decay function. The application of more complex models, such as an exponential 

model (Zuber, 1986) or a binary mixing model (Plummer 

and Busenberg, 2000), which were applied to springs 

discharging from the Floridan aquifer system in the 

Suwannee River basin (Katz and others, 1999), could give 

a better understanding of the flow system.

Table 6. Dissolved gas data and summary of sulfur hexafluoride data for water samples collected at Silver Springs, January 29, 2002

[N2, elemental nitrogen; Ar, argon; O2, oxygen; CO2, carbon dioxide; CH4, methane; SF6, sulfur hexafluoride. oC, degrees Celcius; mg/L, milligrams per liter; 

cc STP/L, cubic centimeters at standard temperature and pressure, per liter; P(SF6), partial pressure of SF6; pptv, parts per trillion by volume; sl, sample lost]

Site Site 

name

Field 

temperature 

(

o

C)

N2

(mg/L)

Ar

(mg/L)

O2

(mg/L)

CO2

(mg/L)

CH4

(mg/L)

Excess 

air

(cc STP/L)

P(SF6) 

(pptv)

Model SF6

recharge 

year

Model SF6

age 

(years)

02239500 Main 23.0 16.851 0.5639 1.49 14.57 0.0000 2.9 1.755 1986.5 15.5

02239500 Main 23.0 sl sl sl sl sl sl 1.790 1987.0 15

291253082030600 Abyss 23.7 15.959 .5545 1.30 12.23 .0000 1.7 3.203 1993.5 8.5

291253082030600 Abyss 23.7 15.939 .5531 1.49 11.91 .0000 1.7 3.045 1993.0 9

291255082030000 Blue Grotto 23.7 16.161 .5558 1.63 10.85 .0000 2.0 3.794 1996.0 6

291255082030000 Blue Grotto 23.7 16.079 .5565 1.62 10.88 .0000 1.8 3.717 1996.0 6

Figure 22. Concentrations of dissolved argon and N2 in water samples from Silver Springs compared to equilibrium 

concentrations of air in water.
38 Chemistry of Ground Water in the Silver Springs Basin, Florida, with an Emphasis on Nitrate

Dating with SF6 can in some instances be complicated by the presence of natural background sources of 

SF6 from rocks or by addition of excess air during 

recharge. Excess air values for the samples were low 

(table 7). Rock sources include fluorite and dolomite 

(Busenberg and Plummer, 2000). Phosphate deposits in 

the Hawthorn Group might be a potential source of fluorite; however, SF6 contamination related to mineralogic 

conditions in north-central Florida has not been documented. Dolomites are present in deeper zones of the 

Floridan aquifer system; the SF6 content of those dolomites is unknown. If the dolomites contain excess SF6, 

mixing with even a small fraction of water from a deeper 

zone could account for SF6 concentrations that result in 

anomalously younger ages.

 Dating of water with 3H/ 3He is possible because 3H 

that was released into the atmosphere by nuclear weapons 

testing during the 1950s and 1960s decays to 3

He with a 

half-life of 12.43 years (Schlosser, 1992). For the 

purposes of dating ground water, tritiogenic helium 

(

3

Hetrit) is estimated from the total 3He by taking into 

account terrigenic (earth-derived) 3

He from minerals in 

the earth’s crust or mantle and 3

He in equilibrium with 

present-day atmospheric conditions. Neon (Ne) is used as 

an indicator of atmospheric helium to separate 3

Hetrit

from the total 3

He of the water sample (Schlosser, 1992). 

The sample from the Main Spring contained relatively 

high (about 52 percent) terrigenic helium—perhaps from 

uranium in sediments of the Hawthorn Group (Cathcart, 

1992).

Tritium-helium-3 data for the spring-water samples 

(table 7) indicated generally older ages than SF6 data 

(about 27 years for the Main Spring and about 18 years 

for the Blue Grotto); however, the 3

H/3Hetrit age for the 

Abyss (about 10 years) agreed with the SF6 age. The 

greater 3

H/3Hetrit ages for the samples from the Main 

Spring and the Blue Grotto could result from the fact that 

ground water discharging from those springs is a mixture 

of water from both shallow flow paths and deeper flow 

paths containing older water with terrigenic SF6 but no 

bomb tritium. On the other hand, the flow system of the 

Abyss is relatively shallow and rapid and includes no 

mixing with older water. One way to assess if 3

H/3Hetrit

results were affected by mixing of waters from different 

flow paths is to compare the reconstructed original 3

H 

content (3

H at time of recharge) of the spring-water 

samples (which equals the recently measured 3

H content 

plus 3

Hetrit) with the historical records of 3H in rainfall 

(Katz, 2004). Data for rainfall measured at the International Atomic Energy Agency monitoring station at 

Ocala, Fla., were reported by Michel (1989). Values that 

do not plot near the rainfall input curve for the estimated 

year of recharge may indicate the effects of mixing. All of 

the reconstructed points plot near the rainfall curve, 

consistent with the apparent recharge years as estimated 

from the 3

H/3Hetrit data (fig 23).

Even though the initial 3H value for the samples plot 

close to the rainfall curve, mixing cannot be completely 

ruled out, as demonstrated by Aeschbach-Hertig and 

others (1998). Multiple mixing scenarios could serendipitously result in reconstructed 3

H/3Hetrit values that plot 

near the rainfall input curve. The conduit system that 

provides flow to the springs is complex and poorly understood. This is apparent from an examination of tritium 

data for the Main Spring in May and July 1967 reported 

by Faulkner (1973). The values were 150 tritium units 

(TU) in May 1967 (when average discharge was about 

850 ft3

/s) and 25 TU in July 1967 (average discharge 

about 800 ft3

/s, which is approximately the long-term 

average flow). Calculations assuming exponential mixing 

suggest that the observed tritium in the spring water could 

represent a mean age of about 1 to 5 years for the May 

samples but about 50 years for the July sample (L. Niel 

Plummer, USGS, Reston, Va., written commun., 2004). 

Table 7. Summary of tritium-helium age data using piston-flow model for water samples collected at Silver Springs, January 29, 2002.

[TU, tritium units; TU = 1 3H atom per 1018 hydrogen atoms; 1 σ, sigma; 3Hetrit, helium derived from tritium; δHe, delta helium; 4He, helium; Ne, neon; 3H/3He, 

tritium-helium; cc STP/g, cubic centimeters at standard temperature and pressure, per gram]

Site Site 

name

Tritium 

(TU)

Tritium 

error, 1 σ 

(TU)

3

Hetrit

(TU)

δ3He 

(percent)

4

He

(cc STP/g)

Ne

(cc STP/g)

Percent 4

He 

terrigenic

3

H/3He 

age

(years)

3

H/3He age 

error

(years)

Apparent

recharge 

year

02239500 Main 3.4 0.09 12.19 -31.93 1.1067x10-7 2.1205x10-7 51.8 27.4 1.3 1974.6 +1.3

291253082030600 Abyss 2.7 .08 1.89 4.27 6.127x10-8 2.3892x10-7 .3 9.9 .6 1992 + 0.6

291255082030000 Blue Grotto 2.7 .09 7.72 4.27 5.574x10-8 1.9959x10-7 10.7 18.9 .5 1983 + 0.5
Chemistry of Ground and Spring Water 39

Such large variations in 3H concentrations in spring water 

over a narrow discharge range are unusual. If the 3

H 

values reported by Faulkner (1973) are correct, this indicates that the age distribution of spring water can shift 

rapidly from mostly young to mostly older water with 

only small fluctuations in total spring discharge.

CFC data could not be used for dating the spring 

water because all samples had CFC concentrations that 

were higher than atmospheric concentrations of CFCs 

(table 8). Excessive CFC concentrations generally are 

caused by the influence of wastewater from treatment 

plants or OSDS (Busenberg and Plummer, 1992) or from 

industrial waste sources. The spring water samples also 

contained detectable concentrations of halon and methyl 

chloroform, the sources of which could be industrial 

(G. Casile, USGS, CFC Laboratory, Reston, Va., oral 

commun., 2002). The presence of these chemicals in 

addition to the detections of DEET in spring waters 

confirm that at least some water discharging from Silver 

Springs is affected by anthropogenic activities.

To summarize the results of dating, both SF6 and 

3

H/3Hetrit data indicate that recharge occurred in the early 

1990s for water from the Abyss. Apparently, water from 

that spring follows a relatively simple, shallow flow path 

and is not affected by mixing or dispersion. On the other 

hand, the water discharging from the Blue Grotto and the 

Main Spring may be affected by mixing of waters from 

different flow paths. Additional sampling from the Main 

Spring under differing flow conditions could add to the 

understanding of the flow system, as could collection of 

samples from discrete points in various flow paths within 

the spring cavern. All three springs are affected by the presence of wastewater in the contributing area, as evidenced 

by the presence of DEET and high CFC concentrations in 

all the samples. The complex conduit flow system contributing water to Silver Springs makes it difficult to draw 

conclusions about the time lag between introduction of N 

loads in the basin and nitrate-N concentrations in the spring 

water. However, the fact that water from the spring is relatively young (less than 30 years) indicates it likely is important to minimize N loading within the contributing area of 

30-year travel times or less.

Figure 23. Comparison of measured tritium and tritiogenic helium concentrations of water samples from Silver 

Springs and the tritium concentration of rainfall at Ocala, Florida (from Michael, 1989, and Michael, unpublished 

data).
40 Chemistry of Ground Water in the Silver Springs Basin, Florida, with an Emphasis on Nitrate

Summary and Conclusions 

Silver Springs, in central Marion County, Florida, is 

the largest nontidal spring group in Florida, with an average discharge of 796 cubic feet per second for 1932-2002. 

The spring group (three main springs and numerous 

smaller springs) is an important natural feature, forming 

the headwaters of the Silver River, and supporting a 

diverse ecosystem. It is also a significant recreational, 

cultural, and economic asset for north-central Florida. 

Nitrate concentration in the water from the Main Spring 

has increased with time. Nitrate enrichment of surface 

waters results in the growth of nuisance vegetation and in 

oxygen depletion in the water.

 In much of the basin supplying ground water that 

supports the spring discharge, the limestone of the Floridan aquifer system is at or near land surface, so recharge 

occurs rapidly. Where the Upper Floridan aquifer is 

unconfined—as is the case in most of the basin—nutrients leached at land surface can easily percolate vertically 

into the aquifer. Land-use activities within the Silver 

Springs ground-water basin in Marion County historically have been predominantly agricultural. However, 

rapid population growth in the City of Ocala and 

surrounding rural areas has resulted in major land-use 

changes. A basin-wide evaluation of the occurrence and 

distribution of nitrate in the Silver Springs basin was 

undertaken to enhance understanding of the processes by 

which nutrients enter and move through the ground-water 

flow system. This included the possible effects of past and 

present land-use activities on water chemistry. Chemical 

constituent concentrations in ground water were documented and the possible effects of localized hydrogeologic conditions (that is, unconfined or confined aquifer) 

on nitrate concentrations were assessed.

Water that discharges from Silver Springs originates 

from the Floridan aquifer system in north-central Florida. 

Delineation of the ground-water basin (the area contributing flow to the spring) historically was based on the potentiometric surface of the Upper Floridan aquifer by 

connecting points perpendicular to the potentiometric 

contours. The delineation of the basin can change over 

time as the potentiometric surface fluctuates. Seasonal 

fluctuations of water levels in wells in the area generally 

are about 3 to 5 feet. Computer models provide another 

approach to delineate a ground-water basin. By adding 

particle tracking to a ground-water flow model, the area 

contributing recharge to a spring for a particular range of 

travel times can be delineated. The contributing area for 

Silver Springs based on travel times up to 500 years is 

smaller than the area delineated on the basis of the potentiometric surface, extending farther south but not as far 

west or north.

Table 8. Concentrations of chlorofluorocarbons (CFC) in water samples collected at Silver Springs, 

January 29, 2002.

[

o

C, degrees Celcius; pg/kg, picogram per kilogram. Note: CFC concentrations in all water samples are higher than 

values in equilibrium with 2002 atmospheric conditions; therefore, age calculation not possible]

Site Site 

name

Recharge 

temperature 

(

o

C)

Ampule 

number

Concentration 

in solution, pg/kg 

CFC-11 CFC-12 CFC-113

02239500 Main 23.5 3 1,629 898 166

02239500 Main 23.5 4 1,670 872 159

02239500 Main 23.5 5 1,541 849 166

291253082030600 Abyss 22.6 2 1,541 1,191 1,209

291253082030600 Abyss 22.6 4 1,523 1,158 1,192

291253082030600 Abyss 22.6 5 1,607 1,255 1,257

291255082030000 Blue Grotto 22.8 2 1,445 1,009 1,376

291255082030000 Blue Grotto 22.8 4 1,418 977 1,458

291255082030000 Blue Grotto 22.8 5 1,528 1,075 1,359
Summary and Conclusions 41

Sources of nitrogen in ground water in the Silver 

Springs basin include atmospheric deposition, fertilizers 

used in agricultural and urban areas, and human and 

animal wastes. The total rate of atmospheric deposition 

averaged about 2,000,000 kilograms per year (kg/yr) total 

nitrogen from 1979-2000. Nitrogen production by livestock was about 3,500,000 kg/yr in the mid-1970s and by 

the late 1990s averaged about 2,500,00 kg/yr. Total nitrogen load from agricultural fertilizer use (based on recommended application rates for various crops) averaged 

about 1,000,000 kg/yr. The actual loads of nitrogen reaching the ground water from atmospheric deposition, livestock, and agricultural fertilization are difficult to 

quantify. As little as 10 percent of atmospheric and 

animal nitrogen may reach the ground water. Nitrogen 

reaching the ground water from residential turf fertilization was estimated to have increased from 114,000 kg in 

1977 to 727,000 kg in 1995. The nitrogen load reaching 

the ground water from golf course fertilization in 2000 

was estimated to be 500,000 kg. Nitrogen load to the 

ground water from municipal wastewater and septic tanks 

totaled about 1,000,000 kg in 2000.

During 2000-01, 56 wells in the Silver Springs 

ground-water basin were sampled for major ions, nutrients, and some trace constituents. Selected wells also 

were sampled for a suite of constituents commonly found 

in wastewater and for nitrogen isotopes. Wells were 

selected to represent both confined and unconfined conditions for the Upper Floridan aquifer, as well as a variety 

of land-use types. Data from the present study were 

compared to data collected from 25 wells sampled in 

1989-90. For the 2000-01 data, calcium concentrations 

ranged from 24 to 176 milligrams per liter (mg/L), with 

a median of 72 mg/L; sulfate values ranged from 0.6 to 

400 mg/L, with a median of 20 mg/L. For the 1989-90 

data, the median calcium concentration was 82 mg/L, and 

the median sulfate concentration was 45 mg/L. These 

differences probably resulted because this study included 

more deep wells than the previous study.

Concentrations of dissolved oxygen, nitrite-plusnitrate (nitrate-N), and phosphorus varied widely among 

the ground-water samples collected during this study. 

Dissolved oxygen concentrations ranged from 0.1 to 

8.3 mg/L. Dissolved oxygen generally is high in unconfined areas of the Upper Floridan aquifer but high values 

also occur in confined areas, such as in the southern part 

of the basin. This could be due to local variations in aquifer confinement that are not apparent on a regionallymapped scale. Concentrations of nitrate-N ranged from 

less than the detection limit of 0.02 to 12 mg/L with a 

median of 1.2 mg/L. Concentrations of total orthophosphate ranged from 0.01 to 0.89 mg/L, with a median of 

0.03 mg/L; total phosphorus ranged from 0.02 to 2.7 mg/L 

with a median of 0.04 mg/L. For data from 1989-90, the 

median nitrate-N was 1.04 mg/L, and the maximum was 

3.6 mg/L. Total orthophosphate had a similar range in both 

studies (fig. 18), although the median for the 1989-90 data 

was higher (0.06 mg/L) than for the data from the present 

study (0.03 mg/L). The maximum total P value for the 

1989-90 data was 0.22 mg/L, compared to a maximum of 

2.7 mg/L during the present study. Except for the maximum 

value, total P values during the present study were in the 

same range as those from the previous study (1989-90). 

Median total P for 1989-90 data was 0.05 mg/L compared 

to 0.04 mg/L for the present study.

 To obtain a better understanding of the relation 

between land use and ground-water chemistry, wells were 

grouped according to reported 1995 land use at the site 

and concentrations of nitrate-N were compared. The agricultural land-use areas had the highest median concentration (1.7 mg/L). The highest value of nitrate-N (12 mg/L) 

was found in an area of agricultural land use, although it 

is uncertain whether the high concentration is related to 

land-use activities or to proximity of the septic tank on the 

property. Because fewer wells were located in rangeland 

and forests, those land-use areas were grouped together. 

This group had the lowest median nitrate-N concentration, 0.09 mg/L. The commercial-industrial-recreational 

group had the highest variability in concentration.

Wells in the area contributing recharge to Silver 

Springs within a 10-year travel time also were grouped by 

land-use type. The maximum nitrate-N concentration 

measured in all urban land-use areas occurred within the 

10-year contributing area; the medians for all urban data 

and urban data within the 10-year contributing area were 

nearly the same. For agricultural land-use areas, the 

median values (both with and without the 12 mg/L value) 

were slightly lower in the 10-year contributing area than 

in the entire study area. For rangeland and forest, the 

median value in the 10-year contributing area is higher 

than the entire study area perhaps because there are urbanized areas upgradient of some of the wells in rangeland 

and forest areas within the 10-year contributing area.

To give insight into the sources of nitrogen in ground 

water in the study area, water samples were collected for 

analysis of the ratio of nitrogen isotopes at 37 wells and 

three springs of the Silver Springs group. Values in 

ground-water samples ranged from -0.5 to 11.5 per mil. 

Values less than 6 per mil generally are indicative of the 

effects of artificial (inorganic) nitrogen (fertilizers). 

Values between 6 to 9 per mil represent mixed inorganic 

and organic sources; values greater than 9 per mil are 
42 Chemistry of Ground Water in the Silver Springs Basin, Florida, with an Emphasis on Nitrate

indicative of organic nitrogen (from human or animal 

wastes). The median value for all ground-water samples 

was 4.9 per mil. For wells in urban land use areas, the 

ratio ranged from -0.5 to 10.8 per mil, with a median of 

5.4 per mil. Values for agricultural land use areas ranged 

from 1.9 to 8.9 per mil, with a median of 4.8 per mil. For 

values in rangeland and forested land areas, the range was 

2.2 to 11.5 per mil, with a median of 4.1 per mil. The three 

sites in transportation, communication or utilities land use 

areas had values of -0.2, 2.7, and 5.2, per mil, respectively. The median for urban land uses (5.4 per mil) is 

slightly higher (indicating more organic nitrogen); the 

medians for all groups were within the range of generally 

inorganic sources.

The Main Spring of Silver Springs was sampled four 

times during the study for the nitrogen isotope ratio. Two 

distinct nitrogen isotope ratios were observed: two values 

indicating inorganic nitrogen sources and two values 

indicating mixed sources with a strong influence of 

organic sources. The different 15N/14N ratios measured 

may be because different conduits were sampled as a 

result of unintended differences in placement of the 

sampling pump during the various sampling events, or 

because the flow contribution from various conduits 

supplying springflow may fluctuate with time.

Water samples from 35 wells and three springs of the 

Silver Springs group also were analyzed for a suite of 63 

compounds commonly found in domestic and industrial 

wastewater to gain additional insight into the sources of 

nitrogen in ground water and in water from Silver 

Springs. Compounds found in detergents and personal 

care products, pesticides, and compounds that result from 

industrial processes were included. Some of the 

compounds are known or suspected endocrine disrupters. 

A total of 38 compounds was detected, nearly all in very 

low concentrations. The most frequently detected 

compound was the insecticide N,N-diethyl-meta-toluamide (DEET), which was detected in water from 27 wells 

and all three spring samples. The presence or absence of 

DEET in ground-water samples did not seem to be related 

to land use, but geologic conditions at well sites generally 

did affect the presence or absence of DEET in the ground 

water. DEET appears to be a useful tracer for the presence 

of wastewater.

In January 2002, water samples were collected from 

the Main Spring, the Abyss, and the Blue Grotto of the 

Silver Springs group and analyzed for concentrations of 

dissolved gasses and for chlorofluorocarbons (CFCs), 

sulfur hexafluoride, and tritium/helium-3 for the purpose 

of dating the young fraction of ground water. Apparent 

ground-water ages are based on a piston-flow model, 

which may not adequately represent a more complex flow 

system. Apparent age for water from the Main Spring is 

about 27 years for tritium/helium-3 data compared to 

about 15 years based on sulfur hexafluoride. For the 

Abyss, the tritium/helium-3 age is about 9 years; the 

sulfur hexafluoride age is about 8 years. For the Blue 

Grotto, tritium/helium-3 age is about 19 years, whereas 

the sulfur hexafluoride age is about 6 years. These results 

indicate that the flow system to the Abyss may be simple 

and adequately represented by the piston flow model. The 

flow systems of the Main Spring and the Blue Grotto, 

however, are more complex and likely include water that 

contains terrigenic SF6 and originates from a deeper flow 

zone. CFC data could not be used for dating because all 

spring samples contained higher than equilibrium concentrations of CFCs.

All three springs (the Main Spring, the Abyss, and the 

Blue Grotto) are affected by the presence of wastewater in 

the contributing area, as evidenced by the presence of 

DEET and elevated CFC concentrations in all the 

samples. The complex conduit flow system contributing 

water to Silver Springs makes it difficult to draw conclusions about the time lag between introduction of nitrogen 

loads in the basin and nitrate-N concentrations in the 

spring water. However, the fact that water from the spring 

is relatively young (less than 30 years) indicates it likely 

is important to minimize nitrogen loading within the 

contributing area of 30-year travel times or less.
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48 Chemistry of Ground Water in the Silver Springs Basin, Florida, with an Emphasis on Nitrate
Appendixes
50 Chemistry of Ground Water in the Silver Springs Basin, Florida, with an Emphasis on Nitrate Appendix A. Water-quality data, 2001-2002. [Refer to appendix B for 1995 land-use code and geologic code. Constituents are dissolved unless otherwise indicated; units are milligrams per liter unless otherwise indicated; μS/cm, microsiemens per centimeter; oC, Celsius; μg/L, micrograms per liter; coliform, colonies/100 milliliters; SpC, specific conductance; NH3, ammonia; NH3 + org N, ammonia plus organic nitrogen; NO2 + NO3, nitrite-plus-nitrate or nitrate-N; NO2, nitrite; 15N/14N, nitrogen-15, nitrogen-14, ratio of nitrogen-15 to nitrogen-14; DEET, N,N-diethyl-meta-toluamide. NA, not analyzed; ND, not detected; D, presence of material verified but not quantified; --, no data] Well No. Site Well depth (feet) Date Dissolved oxygen pH, field (standard units) pH, lab (standard units) SpC, lab (μS/cm) SpC, field (μS/cm) Temper- ature

(

oC) 

Calcium Mag- nesium Potas- sium Sodium Alka- linity Chloride Fluoride

1 285823082004101 58 5/7/2002 5.51 7.55 8.0 249 250 24.9 34 4.4 5.6 3.8 81 8.2 0.22 285853082063901 -- 5/14/2002 4.2 7.31 7.7 312 320 23.7 56 3 .3 3.5 142 7 .13 285855082010801 78 5/7/2002 6.66 7.3 7.8 182 180 24.6 25 4.6 .4 2.7 66 5.2 .14 285856081591901 220 5/7/2002 5.35 7.83 8.1 200 200 24.7 24 8 .3 2.6 75 5.2 <.15 285904082040401 90 5/14/2002 8.3 7.45 7.8 246 240 23.9 46 1.1 <.1 2.8 110 4.9 <.16 285954081590901 -- 4/16/2002 4.47 7.57 8.0 248 250 24.1 35 5.9 .5 6.6 106 11 .17 290012082001401 -- 4/16/2002 7.4 7.5 8.1 170 170 24.4 26 3.4 .3 2.4 70 4.5 <.18 290103081580401 -- 4/16/2002 1.35 7.1 7.7 523 516 22.9 79 8.7 4.6 13 184 25 .19 290226082120501 210 5/15/2002 1.0 7.03 7.7 970 970 23.8 176 25 1.4 7.5 128 12 .2

10 290445082022101 -- 4/17/2002 4.3 7.24 7.7 470 500 25.3 72 12 .9 5.3 92 8.3 .111 290456082014001 73 5/1/2002 5.84 9.17 8.8 579 590 23.2 48 4.1 4.5 58 96 88 <.112 290505082012801 70 5/1/2002 4.88 8.49 8.2 234 242 25.1 33 4.57 .7 6.1 66 10.9 .113 290650082053001 110 5/16/2001 2.7 7.5 8.0 481 488 24.1 74 13 .7 5.5 141 8.7 .214 290754082123101 60 5/15/2002 6.3 6.5 7.6 343 350 23.9 65 1.2 .1 2.6 161 6.2 <.115 290810082024001 -- 5/13/2002 5.43 7.21 7.9 284 292 24.7 42 6.1 .5 5.2 101 9.3 .116 290810082063001 75 5/16/2001 .8 7.0 8.0 570 575 22.7 110 2.6 .4 12 249 21 <.117 290815082025701 105 8/14/2001 6.0 7.0 7.7 324 408 24.6 83 1.5 .2 2.2 164 3 .117 290815082025701 105 8/14/2001 6.0 7.0 7.7 318 408 24.6 83 1.4 .2 2.2 160 2.9 .118 290838082103501 -- 5/16/2001 3.3 7.2 7.9 687 694 24.1 120 13 .7 8 149 15 .219 290843082053901 -- 5/15/2001 6.4 7.3 7.8 349 357 23.4 56 3.9 .7 8.3 124 19 .119 290843082053901 -- 5/15/2001 6.4 7.3 7.8 350 357 23.4 56 3.9 .7 8.3 124 19 .120 290850082080001 170 5/15/2001 6.0 7.4 7.7 477 487 24.3 73 8.4 1.2 11 150 15 .221 290850082094001 -- 5/15/2001 5.1 7.2 7.5 649 660 23.9 120 11 1 9 196 15 .222 290900082070001 145 5/15/2001 5.0 7.2 7.6 525 537 23.8 85 11 .5 7.4 190 11 .223 290915082023201 55 8/13/2001 1.0 9.8 8.5 174 234 24.6 32 3.3 1.7 5.6 29 8.2 .224 290959082114501 65 5/14/2002 5.7 7.45 7.8 502 -- 25.5 74 9.5 .7 10 101 17 .225 291057082033401 115 5/14/2001 5.5 7.5 7.8 345 350 23.2 51 7.7 .5 5.9 113 9.4 .126 291105082005901 -- 5/16/2001 -- 7.6 8.2 475 481 23.5 69 14 1.2 7.8 135 10 .227 291110082060001 91 8/15/2001 4.8 6.9 7.8 369 458 25.2 90 2.6 .6 3 193 3.4 .228 291121082044001 117 5/10/2001 5.8 7.3 7.8 468 450 23.8 71 11 .7 6.7 152 10 .229 291123082065001 57 8/14/2001 .8 6.9 7.8 383 426 24.6 81 3 1 3.1 176 4.6 .230 291123082075402 70 8/15/2001 .4 6.3 7.4 378 368 24.7 53 7.7 1.1 13 136 16 .431 291124082070201 250 5/9/2001 3.6 7.1 7.3 628 630 23.4 110 11 1 12 243 16 .2
Appendix A 51

Appendix A. Water-quality data, 2001-2002. (Continued)

Well 

No. Silica Sulfate NH3 NH3 + org N NO2 + NO3 NO2 Phos- phorus, dissolved Ortho- phos- phate Phos- phorus, total Fecal coliform Boron (μg/L) Iron (μg/L) Manga- nese (μg/L) 15N/14N (per mil) DEET (μg/L) 1995 land use code Geologic code

1 4.3 7.9 <.010 0.2 6.3 <.010 <.020 0.03 0.54 NA 11 <2 <1.0 2.6 D 2110 TQu2 6.6 6.7 <.010 .3 1.8 <.010 .02 .02 .02 NA 13 2.9 <1.0 NA D 1100 To3 11 5.1 <.010 <.2 4 <.010 .09 .1 .11 NA 10 <2 <1.0 2.7 1.70 8320 TQu4 9.9 13 .24 <.2 1.6 .01 <.020 .02 <.02 NA 10 20 3.6 3.3 ND 1100 TQu5 5.2 1.5 <.010 <.2 2.4 <.010 <.020 <.01 <.02 NA 11 <2 <1.0 1.9 D 2110 To6 12 2.4 .01 <.2 1.4 <.010 .04 .05 .02 NA 17 <2 <1.0 5.8 ND 1700 Tc7 7.9 6.3 .02 <.2 1.4 <.010 <.020 .02 <.02 NA 9 2.1 <1.0 3.6 ND 2150 TQu8 9.4 46 .15 <.2 2 .03 <.020 .02 <.02 NA 51 70 16 11.5 ND 4430 Tc9 12 400 .06 .3 <.02 <.010 <.020 .01 <.02 NA 28 230 14 3.6 D 4120 To

10 10 140 .02 <.2 1.2 <.010 .03 .04 .02 NA 16 10 <1.0 2.4 D 2610 TQu11 8.5 24 <.010 <.2 9.9 .02 .04 .05 .04 NA 74 <2 <1.0 8.9 D 2150 TQu12 8.6 28.5 <.010 <.2 2.2 <.010 .03 .03 .03 NA 12 <2 <1.0 5.2 D 8320 TQu13 11 93 <.010 .2 1.1 <.010 .03 .03 .03 <1 20 <2 <1.0 4.8 NA 2130 To14 8.5 1.3 <.010 <.2 .93 <.010 .03 .03 .03 NA 8 <2 <1.0 2.2 D 4340 To15 9.1 25 <.010 <.2 1.1 <.010 .03 .02 .03 NA 15 <2 <1.0 5.4 ND 1100 TQu16 11 5.3 .144 .24 5.1 <.010 .09 .09 .09 <1 20 2.1 <1.0 8.2 NA 1550 Thc17 8.4 3.9 <.010 <.2 .94 <.010 .03 .02 .13 NA 12 2.3 <1.0 -.2 D 8140 Thc17 8.6 3.8 <.010 <.2 .88 <.010 .03 .02 .1 1 11 2.6 <1.0 NA D 8140 Thc18 9.6 180 <.010 <.2 2.7 <.010 .03 .03 .03 <1 20 10 <1.0 6 NA 2510 Thc19 11 17 <.010 <.2 2.2 <.010 .03 .03 .03 <1 20 <2 <1.0 NA NA 1100 Thc19 11 17 <.010 <.2 2.2 <.010 .03 .03 .03 <1 20 2.6 <1.0 5.2 NA 1100 Thc20 14 68 <.010 <.2 2.2 <.010 <.020 .02 <.02 <1 30 4.0 <1.0 6.1 NA 3100 Thc21 9.9 120 <.010 <.2 2.4 <.010 .02 .03 .02 <1 20 40 1.1 5 NA 2510 Thc22 10 74 <.010 <.2 1.2 <.010 .02 .03 .02 <1 30 3.5 <1.0 NA NA 1410 Thc23 9.5 29 2.1 2.1 .85 .04 <.020 <.01 .04 <1 15 4.8 <1.0 7.5 D 2110 TQu24 9 120 <.010 <.2 1.4 <.010 <.020 .01 <.02 NA 18 8.3 1.5 6.3 D 1100 To25 9.3 41 <.010 <.2 1.7 <.010 <.020 .03 .02 <1 20 <2 <1.0 NA NA 1100 Thc26 15 95 .056 <.2 <.02 <.010 <.020 .01 <.02 <1 20 40 3.5 NA NA 3200 TQu27 8.8 21 .01 <.2 .37 <.010 <.020 .01 .02 <1 15 <2 <1.0 -.5 D 1410 Qu28 10 72 <.010 <.2 1.5 <.010 .02 .04 .03 <1 20 2.3 <1.0 NA NA 1200 Thc29 6.6 18 <.010 <.2 <.02 <.010 .03 .03 .07 1 16 <2 <1.0 10.8 D 1200 Thc30 15 32 .06 <.2 .38 <.010 .89 .89 2.7 19 26 60 41 10.8 5.80 1850 Thc31 12 64 <.010 <.2 1.1 <.010 <.020 .03 <.02 <1 30 5.3 <1.0 NA NA 1200 Thc
52 Chemistry of Ground Water in the Silver Springs Basin, Florida, with an Emphasis on Nitrate Appendix A. Water-quality data, 2001-2002. (Continued) Well No. Site Well depth (feet) Date Dissolved oxygen pH, field (standard units) pH, lab (standard units) SpC, lab (μS/cm) SpC, field (μS/cm) Temper- ature

(

oC) 

Calcium Mag- nesium Potas- sium Sodium Alka- linity Chloride Fluoride

32 291134082031401 110 5/10/2001 3.7 7.6 8.0 373 360 24.4 55 8.4 .6 6.9 117 11 .133 291148082072702 75 8/15/2001 3.1 6.8 8.0 584 669 24 94 13 .6 29 264 31 .434 291205082030301 120 5/10/2001 1.3 7.4 8.0 325 3 23.5 47 9.2 .5 4.3 112 6.2 .235 291226082042001 -- 5/10/2001 .6 7.5 7.9 530 525 24.7 82 13 .8 7.5 152 13 .236 291235082031901 -- 8/16/2001 3.0 7.1 8.4 495 514 23.9 80 12 .7 6 165 9.8 .237 291239082060401 -- 5/17/2001 2.7 7.2 7.9 493 496 23.7 83 9.5 2.2 6.8 191 10 .238 291248081582401 -- 3/26/2002 .1 7.5 8.2 312 340 23.5 39 11 1.3 8.4 152 5.3 .439 291303082021401 -- 8/15/2001 -- 7.0 8.1 773 805 22.5 126 20 1.8 16 226 24 .340 291314082024201 130 5/9/2001 1.1 6.9 7.8 349 345 22.5 57 6 .5 5.6 146 9 .141 291320082042301 100 5/8/2001 5.5 7.7 8.0 191 185 23.4 34 2.2 .2 2.6 91 4 <.142 291321082072401 125 5/8/2001 2.5 7.3 7.6 455 445 24 76 10 .5 5.3 208 8.8 .243 291337082043801 165 5/10/2001 .7 7.4 8.0 364 350 23.1 61 6 .5 5.1 162 8.1 .144 291341082003401 170 5/14/2001 .2 7.5 7.8 829 863 22.9 120 25 2.2 20 206 34 .445 291400082070001 70 5/9/2001 5.5 7.1 7.4 578 550 22.6 92 3.2 .2 26 247 24 <.146 291424082094701 -- 3/26/2002 4.0 7.0 8.0 466 243 23.6 88 3.8 .3 6.5 229 8.9 .147 291529082132001 120 5/8/2002 1.98 7.09 7.6 341 339 22.7 62 2 .3 4.9 160 9.7 .148 291718082064201 -- 3/26/2002 5.5 6.8 8.1 1,270 1,300 24.6 113 4.9 .3 140 272 230 <.149 291723082051601 -- 5/22/2002 6.88 7.49 7.9 356 355 23.5 56 2.5 1.2 10 102 20 <.150 291823082051801 80 5/22/2002 1.65 7.5 7.9 272 272 22.9 49 3 .2 4.3 126 8.3 .151 291931082130401 155 5/8/2002 4.79 6.86 7.3 474 470 22.6 86 4.1 .5 8.6 225 11 .252 292003081452001 -- 3/26/2002 5.6 7.9 8.1 423 420 23.3 26 12 1.3 33 46 65 <.153 292020082064801 -- 3/25/2002 1.8 7.1 7.8 353 360 24.2 64 2.3 .3 6.5 168 9.6 <.154 292135082051901 84 5/21/2002 .24 6.99 7.3 470 466 22.8 95 2.1 .6 4.3 242 8.9 <.155 292205082022901 145 5/21/2002 .15 7.04 7.5 447 445 21.9 90 1.7 .2 5.1 230 7.9 .356 292320082170101 130 5/8/2002 .78 6.72 7.2 549 500 21.8 112 1.8 .1 3 285 6.5 .2

Springs

291253082030600 Abyss 11/15/01 a 3.7 7.2 -- 500 468 23.6 74 12 .7 6.4 157 9 .16291255082030000 Blue Grotto 11/15/01 a 3.2 7.3 -- 480 443 23.5 70 11 .7 6.5 153 9 .1502239500 Main 11/15/01 a 2.4 7.2 -- 510 471 23.2 76 11 .7 6.9 176 9.2 .17Main (USGS data) 05/08/01 1.3 6.7 -- -- 451 23.2 -- -- -- -- -- -- --Main 01/29/02 1.5 7.4 -- -- 458 23.0 -- -- -- -- -- -- --Main 07/18/02 2.7 6.4 -- -- 466 23.3 -- -- -- -- -- -- --Main 08/14/02 2.8 7.0 7.6 462 474 23.2 76 9.5 .6 6.3 189 10 .2

[Refer to appendix B for 1995 land-use code and geologic code. Constituents are dissolved unless otherwise indicated; units are milligrams per liter unless otherwise indicated; μS/cm, microsiemens per 

centimeter; oC, Celsius; μg/L, micrograms per liter; coliform, colonies/100 milliliters; SpC, specific conductance; NH3, ammonia; NH3 + org N, ammonia plus organic nitrogen; NO2 + NO3, nitrite-plus-nitrate 

or nitrate-N; NO2, nitrite; 15N/14N, nitrogen-15, nitrogen-14, ratio of nitrogen-15 to nitrogen-14; DEET, N,N-diethyl-meta-toluamide. NA, not analyzed; ND, not detected; D, presence of material verified but 

not quantified; --, no data]
Appendix A 53

Appendix A. Water-quality data, 2001-2002. (Continued) 

Well 

No. Si SO4 NH3 NH3 + org N NO2 + NO3 NO2 Phos- phorus, dissolved Ortho- phos- phate Phos- phorus, total Fecal coliform Boron (μg/L) Iron (μg/L) Manga- nese (μg/L) 15N/14N (per mil) DEET (μg/L) 1995 land use code Geologic code

32 9.8 50 <.010 <.2 1.8 <.010 .02 .04 .02 <1 20 6.6 <1.0 NA NA 1200 Thc33 11 23 .02 <.2 .84 <.010 .06 .06 .11 <1 29 <2 <1.0 4.5 D 1200 Thc34 9.5 46 <.010 <.2 .09 <.010 <.020 .03 <.02 <1 10 50 1.9 NA NA 4120 Qu35 7.8 100 <.010 .24 1.1 <.010 <.020 .02 .09 <1 20 820 530 NA NA 1410 Qu36 10 84 <.010 <.2 1.2 <.010 .02 .03 .02 <1 18 5.4 <1.0 4.4 ND 4120 Qu37 11 49 .018 <.2 1.2 <.010 .04 .04 .04 <1 20 3.1 <1.0 4.9 NA 1410 Qu38 44 7.2 .2 <.2 <.02 <.010 <.020 <.01 <.02 NA 16 10 2.9 NA D 4110 Tc39 18 160 .11 .3 <.02 <.010 <.020 .02 .02 <1 32 290 8.8 NA ND 4340 Qu40 8.3 20 <.010 <.2 .6 <.010 .04 .05 .04 <1 10 2.1 <1.0 NA NA 1700 Qu41 8.1 2.6 .016 <.2 .42 <.010 <.020 <.01 <.02 <1 20 <2 <1.0 NA NA 1550 Qu42 13 18 .016 <.2 1 <.010 .03 .03 .04 <1 20 <2 <1.0 NA NA 1200 To43 7.8 15 <.010 <.2 .67 <.010 .02 .05 .03 <1 10 <2 <1.0 NA NA 1920 Qu44 17 200 .068 .22 .2 <.010 <.020 <.01 <.02 <1 40 960 60 NA NA 1100 Qu45 9 16 <.010 <.2 1.7 <.010 .05 .05 .05 <1 20 4.5 1.8 NA NA 2510 To46 8.2 7 <.010 <.2 3.1 <.010 <.020 .03 <.02 NA 17 <2 <1.0 6.5 D 1200 To47 10 1.5 <.010 .3 .77 <.010 <.020 .02 <.02 NA 14 <2 <1.0 3.8 D 4340 To48 8.6 16 <.010 <.2 5.9 <.010 .06 .07 .06 NA 16 20 <1.0 7.3 D 1700 Thc49 6.9 0.6 <.010 <.2 12 <.010 <.020 .02 <.02 NA 11 <2 <1.0 7.1 D 2510 Thc50 5.8 1.4 .02 <.2 .65 <.010 <.020 <.01 <.02 NA 11 6.1 1.3 6.1 D 1110 To51 17 7.5 <.010 <.2 1.2 <.010 .09 .09 .09 NA 21 <2 <1.0 4.3 D 2510 Thc52 12 52 <.010 <.2 .32 <.010 <.020 <.01 <.02 NA 18 20 <1.0 4.1 D 1100 Thc53 7.5 1.2 <.010 <.2 1.5 <.010 <.020 .01 <.02 NA 12 <2 <1.0 4.6 ND 1700 Thc54 5.5 <.2 .99 1.3 <.02 <.010 .13 <.01 .14 NA 18 2170 28 3.3 D 1100 To55 14 1.4 .02 <.2 <.02 <.010 <.020 .01 .26 NA 14 310 6.1 NA D 4410 To56 16 1.8 <.010 <.2 .05 <.010 .05 .06 .05 NA 13 30 1.7 NA D 2110 Thc

SpringsAbyss -- 74 -- -- 1.4 -- -- -- .038 <25 <35 <1 4.9 0.68 b

Blue -- 64 -- -- 1.4 -- -- -- .039 <25 <35 <1 5.8 D b

Main -- 60 -- -- 1.1 -- -- -- .044 <25 <35 <1 3.8 lostMain -- -- -- .8 -- -- -- 8.7 NAMain -- <.01 <.2 .81 <.01 .03 .05 .03 NA NA NA NA 8.1 1.2Main -- <.01 <.2 .93 <.01 .04 .04 .05 NA NA NA NA 3.7 DMain -- 43 NA .9 1.0 c .05

a Sample was analyzed by the Florida Department of Environmental Protection, except for 15N/14N.b November sample lost. This is result of resampling on January 29, 2002.c Indicates filtered sample.
Appendix B. Land-use and geologic codes.

[Source of 1995 land-use codes: St. Johns River Water Management District, 1999; source of geologic 

codes: Scott and others, 2001]

1995 Land-Use Codes (LUCODE)

Urban: 

1000-1199 Low-density residential (less than 2 dwelling units per acre)

1200-1399 Medium and high-density residential (more than 2 dwelling units per acre)

1400-1799 Commercial/Industrial

1800-2000 Recreation/Open land

Agricultural: 

2110-2133 Pasture

2140-2160 Croplands

2300-2333 Livestock operations

2510 Horse farms

2610 Fallow croplands

Rangeland:

3000-3999 Rangeland

Upland forests:

4000-4999 Upland forests

Water:

5000-5999 Water

Wetlands:

6000-6999 Wetlands

Barren lands:

7000-7999 Barren lands

Transportation, communications, and utilities:

8100 Transportation

8200 Communications

8300 Utilities

8340 Sewage treatment plants

Geologic Codes

Tc Tertiary Cypresshead Formation

Thc Tertiary Hawthorn Group

To Tertiary Ocala Limestone

TQu Tertiary-Quaternary undifferentiated sediments

Qu Quaternary undifferentiated sediments

54 Chemistry of Ground Water in the Silver Springs Basin, Florida, with an Emphasis on Nitrate