U.S. Department of the Interior

U.S. Geological Survey

Open-File Report 2014–1255

Prepared in cooperation with the City of Tallahassee

Depth-Dependent Groundwater Quality Sampling 

at City of Tallahassee Test Well 32, Leon County, 

Florida, 2013
Cover. Well drillers installing submersible pump in well 32. Photograph by W. Scott McBride, USGS.
Depth-Dependent Groundwater Quality Sampling 

at City of Tallahassee Test Well 32, Leon County, 

Florida, 2013

By W. Scott McBride and Michael A. Wacker

Prepared in cooperation with the City of Tallahassee

Open-File Report 2014–1255

U.S. Department of the Interior

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

SALLY JEWELL, Secretary

U.S. Geological Survey

Suzette M. Kimball, Acting Director

U.S. Geological Survey, Reston, Virginia: 2015

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Suggested citation:

McBride, W.S., and Wacker, M.A., 2015, Depth-dependent groundwater quality sampling at City of Tallahassee test 

well 32, Leon County, Florida, 2013: U.S. Geological Survey Open-File Report 2014–1255, 13 p. and 2 appendixes, 

http://dx.doi.org/10.3133/ofr20141255.

ISSN 2331-1258 (Online)
iii

Contents

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

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

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

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

Methods of Study...........................................................................................................................................2

Borehole-Geophysical Data Collection.............................................................................................2

Collection and Analysis of Water Samples ......................................................................................3

Source Tracers .....................................................................................................................................3

Age Dating..............................................................................................................................................4

Results .............................................................................................................................................................4

Borehole-Geophysical Data................................................................................................................4

Water-Quality Data...............................................................................................................................5

Source Tracers .....................................................................................................................................5

Age Dating............................................................................................................................................10

Summary and Conclusions ........................................................................................................................11

References Cited .........................................................................................................................................12

Appendix 1. Comparison of borehole image, geophysical, water quality, flowmeter, 

and sonic logs showing evidence of three productive intervals in the City of 

Tallahassee test well 32 at Leon County, Florida, December 2013. Only the 

data collected in the open interval of the test well are displayed at 1 to 96 scale .............13

Appendix 2. Comparison of borehole image, geophysical, water quality, flowmeter, 

and sonic logs showing evidence of three productive intervals in the City of 

Tallahassee test well 32 at Leon County, Florida, December 2013. The full 

geophysical log is displayed at 1 to 12 scale.............................................................................13
iv

Figures

1. Map showing City of Tallahassee test well 32 in Leon County, Florida ...............................1

2. Stiff diagrams comparing the major ion concentrations of four water samples 

collected at the City of Tallahassee test well 32, December 2013........................................7

3. Graph showing content of delta deuterium and delta oxygen-18 in water samples 

collected from the City of Tallahassee test well 32 relative to the global meteoric 

water line in December 2013 ......................................................................................................7

4. Graph showing relation between strontium isotope ratios and the inverse of the

strontium concentration in water samples collected at the City of Tallahassee

test well 32, December 2013 .....................................................................................................10

Tables

1. Major ions, nutrients, and trace elements in groundwater samples collected

at the City of Tallahassee test well 32, December 2013 .........................................................6

2. Volatile organic compounds in groundwater samples collected from

City of Tallahasse test well 32, December 2013.......................................................................8

3. Field measurements, tracer, and age data collected from City of Tallahassee

test well 32, December 2013 .....................................................................................................11
v

Conversion Factors and Datums

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) 2.590 square kilometer (km2) 

Flow rate

gallon per minute (gal/min) 0.06309 liter per second (L/s)

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

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 is referenced to the North American Datum of 1983 (NAD 83).

Altitude, as used in this report, refers to distance above the vertical datum.

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

25 °C). 

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

or micrograms per liter (μg/L).

Abbreviations

ABI acoustic borehole imager

bls below land surface

cps counts per second

CW32 test well 32

δ18O delta oxygen-18

δ2H delta hydrogen-2

EM electromagnetic 

2

H deuterium

3

H tritium

KHz kilohertz

OBI optical borehole imager

per mil parts per thousand

SF6 sulfur hexafluoride

86Sr strontium-86

87Sr strontium-87

SFM spinner flowmeter

TU tritium units

USGS U.S. Geological Survey

WQT water-quality tool
vi

Acknowledgments 

The authors would like to thank Dr. Brian Katz, retired U.S. Geological Survey and currently 

Florida Department of Environmental Protection, and David Roberts, City of Tallahassee, who 

originally conceived of this investigation. Their advice and consultation during the planning and 

execution of the study are much appreciated. 

The authors also thank Dr. Kim Haag, Reports Specialist for the USGS Caribbean-Florida 

Water Science Center, for her substantial contributions in preparing this report for publication. 

Dr. Haag reviewed, revised, and formatted the report with great efficiency and her efforts 

are appreciated.
Depth-Dependent Groundwater Quality Sampling 

at City of Tallahassee Test Well 32, Leon County, 

Florida, 2013

By W. Scott McBride and Michael A. Wacker

Abstract 

Public-supply wells sometimes produce water of less 

than desirable quality because contaminants can migrate to the 

open interval of wells through preferential pathways. If these 

pathways can be identified, zones that produce poor quality 

water can be excluded during the well-construction process. 

The U.S. Geological Survey has developed geophysical 

testing methods that can be used to delineate zones of high 

permeability in test wells. Once the highly permeable zones 

are identified, water-quality data can be collected from each 

zone to identify whether any of the zones produce water of 

poor quality. The zones producing poor quality water can 

then be cased off in the final well design so that they do not 

contribute flow to the production well, reducing subsequent 

water-treatment costs. 

A test well was drilled by the City of Tallahassee to 

assess the suitability of the site for the installation of a 

new well for public water supply. The test well is in Leon 

County in north-central Florida. The U.S. Geological Survey 

delineated high-permeability zones in the Upper Floridan 

aquifer, using borehole-geophysical data collected from the 

open interval of the test well. A composite water sample was 

collected from the open interval during high-flow conditions, 

and three discrete water samples were collected from specified 

depth intervals within the test well during low-flow conditions. Water-quality, source tracer, and age-dating results 

indicate that the open interval of the test well produces 

water of consistently high quality throughout its length. The 

cavernous nature of the open interval makes it likely that the 

highly permeable zones are interconnected in the aquifer by 

secondary porosity features.

Introduction 

Public-supply wells are vulnerable to water-quality issues 

due to the movement of contaminants through short-circuit 

pathways that can be induced by pumping and other processes 

(Landon and others, 2009). Large costs can be associated with 

the installation and maintenance of a public-supply well used 

to provide a clean, safe, and reliable source of drinking water. 

Treatment costs can be minimized if the spatial distribution 

of zones of substantial flow and high-quality water can be 

delineated in test wells prior to the construction of a publicsupply well. 

The City of Tallahassee operates 28 water-supply wells 

throughout the city and surrounding areas that draw water 

from the Upper Floridan aquifer (Davis and Katz, 2007). 

In general, high-quality groundwater is being withdrawn; 

however, problems have been identified in a few of the wells, 

with slightly elevated concentrations of iron, manganese, 

arsenic, nitrate, and tetrachloroethylene. To meet a growing 

demand for additional public water supply, the City of 

Tallahassee plans to complete a new public-supply well north 

of the city. Test well 32 (CW32) was drilled to confirm the 

suitability of the water in the open interval for public water 

supply (fig. 1). 

The U.S. Geological Survey (USGS) has developed 

depth-dependent sampling techniques that can be used 

to collect water-quality samples and flow-rate data from 

discrete depth intervals within public-supply wells and 

boreholes during pumping and non-pumping conditions. 

Location of

test well 32 in

Leon County 

Figure 1. Map showing City of Tallahassee test well 32 in 

Leon County, Florida.

N

F L O R

I

D

A

Figure 1. City of Tallahassee test well 32 in Leon County, Florida.
2 Depth-Dependent Groundwater Quality Sampling at City of Tallahassee Test Well 32, Leon County, Florida, 2013

Depth-dependent flow and water-chemistry profiles in pumped 

wells are useful for understanding sources and pathways for 

contaminants to supply wells (Collar and Mock, 1997; Gossell 

and others, 1999; Izbicki and others, 1999, 2005, 2008; 

Danskin and Church, 2005). 

Purpose and Scope

The purpose of this report is to present the findings of a 

study conducted by the USGS, in cooperation with the City 

of Tallahassee, to (1) delineate high-permeability zones in the 

Upper Floridan aquifer at test well CW32 that have substantial 

rates of groundwater flow and (2) to characterize changes in 

water quality with depth in the CW32 borehole. Delineation 

of the high-permeability zones in the Upper Floridan aquifer 

was determined using a suite of geophysical logging tools. 

Composite water samples were collected from the open 

interval, and discrete depth-dependent samples were collected 

from each of the three productive zones in CW32. Data 

were collected during December 2013. Water samples were 

analyzed for a variety of water-quality constituents including 

major ions, nutrients, dissolved organic carbon, trace elements, 

volatile organic compounds; natural tracers including isotopes 

of oxygen, hydrogen, and strontium; and age-dating indicators 

including tritium and sulfur hexafluoride.

Description of the Study Area

The study area is in Leon County in north-central Florida 

(fig. 1). The topography of the study area is characterized 

by rolling hills and land-surface altitudes that range from 

0 to about 200 feet above the National Geodetic Vertical 

Datum of 1929. The climate is humid, subtropical with 

relatively large amounts of rainfall. The average annual 

temperature in Tallahassee is 67 degrees Fahrenheit, and 

the annual average precipitation is about 66 inches per year. 

The city water-supply wells are widely distributed, and all 

28 existing wells are interconnected through distribution 

lines. City of Tallahassee test well 32 is located approximately 

9 miles north of downtown Tallahassee, about 2.5 miles west 

of Thomasville Road in the Summerbrooke Subdivision.

Methods of Study

Borehole-geophysical data were collected in the open 

interval of CW32 to delineate zones of high permeability and 

flow. Water chemistry data were collected and used to determine the quality of water produced by the high-permeability 

zones in CW32. Naturally occurring chemical tracers were 

used to determine whether the high-flow zones in CW32 yield 

water from similar sources. Transient age-dating tracers were 

used to determine if water from each of the highly permeable 

zones in CW32 is of similar age.

Borehole-Geophysical Data Collection

The total depth of the CW32 borehole drilled by the 

City of Tallahassee is 440 feet (ft) below land surface (bls), 

with a casing depth of 300 ft and a borehole diameter of 

10 inches (in.). Prior to the collection of borehole-geophysical 

data, the borehole was bridged or collapsed below 413 ft 

bls, and therefore, borehole data were collected between 

the base of the casing and 413 ft (natural gamma ray log 

data were collected from the land surface to 413 ft bls). 

Borehole-geophysical data from CW32 were collected in two 

stages on December 2–3, 2013. On December 2, continuous 

digital borehole images, fluid, caliper, natural gamma, 

electromagnetic-induction, and acoustic velocity data were 

collected. WellCAD version 4.4 was used to plot the borehole 

data and to display the data at different scales (1 to 96 and 

1 to 12 scales) to better depict the range of data in the discrete 

well intervals. This data display was used to plan the subsequent collection of flowmeter data on the following day.

Continuous digital borehole images were obtained using 

high-resolution optical borehole image (OBI) and an acoustic 

borehole image (ABI) devices. The ABI data supplement 

data from the OBI by displaying physical features that may 

be difficult to distinguish optically (Williams and Johnson, 

2004). In addition, the amplitude of the acoustic signal reflects 

the density and cementation of the rock in the borehole wall 

(Prensky, 1999; Williams and Johnson, 2004). 

Fluid data were collected in the open section of the 

borehole under ambient flow conditions, using two instruments: (1) a water-quality tool (WQT) that measures fluid 

temperature, specific conductance, pH, percent oxygen, and 

oxidation-reduction potential (redox) of the borehole fluid 

and (2) a fluid probe on the end of a caliper tool that measures 

borehole-fluid temperature and resistivity. 

Other borehole-geophysical data were collected using a 

three-arm caliper probe, natural gamma tool, electromagnetic 

(EM) induction tool, and acoustic velocity tool. The threearm caliper probe was used to measure the diameter of the 

borehole with depth and to locate zones of potential secondary 

porosity (Keys, 1990). Natural gamma data typically are used 

to correlate changes in lithology and stratigraphy between 

boreholes (Keys, 1990). An electromagnetic induction tool 

was used to provide bulk electrical conductivity data for 

the aquifer rock matrix and formation water. These data 

are influenced by formation porosity, groundwater specific 

conductance, and temperature (Wacker and others, 2014). 

Formation resistivity is the reciprocal of conductivity, so EM 

induction data are comparable to resistivity data (Keys, 1990). 

Acoustic velocity logs were recorded using full waveform 

sonic data collected over the open-hole section in two logging 

runs with one acoustic signal transmitted at 15 kilohertz 

(KHz; data used for porosity calculations) and a second signal 

transmitted at 1 KHz (data used for relative permeability).

On December 3, 2013, a spinner flowmeter (SFM) was 

used, under both ambient and pumping conditions, to measure 

the relative change in flow as the tool was moved up and down 
Methods of Study 3

the borehole. The SFM measures flow using a propeller that 

spins faster or slower depending on the rate of vertical flow 

within the borehole. Depths where changes in spin velocity 

occur, measured in counts per second (cps), indicate locations 

where inflow and outflow of groundwater to the borehole 

occurs. Heat-pulse and electromagnetic flowmeters, which 

measure the actual velocity of flow in the well bore, were 

not used because these meters require the borehole to be of a 

uniform diameter. Heat-pulse and electromagnetic flowmeters 

require that water be routed through a central measuring 

orifice by way of a rubber diverter. When the borehole wall 

is not uniform, flow can escape around the diverter, biasing 

the measurement. The SFM can be used in boreholes that are 

cavernous, like CW32, because it does not require that flow be 

confined in order for a measurement to be obtained. 

The SFM was first logged up and down the open borehole 

under ambient flow conditions. After the ambient logging was 

completed, an electrical submersible pump was installed in 

the well at 230 ft bls. This was done while the SFM was still 

in the borehole, because there was insufficient space in the 

well casing for the pump and SFM to pass one another. The 

SFM was then moved up and down the borehole as the pump 

was run at different speeds to measure the vertical flow in the 

borehole under varying pumping conditions. Fluid data were 

also collected with the WQT after pumping was completed 

(and the pump removed) to determine if the borehole fluid had 

returned to ambient pre-pumping conditions. The WQT was 

not used during pumping because it would have required the 

removal of the pump from the test well. 

Collection and Analysis of Water Samples

Composite and discrete depth-dependent water samples 

were collected at CW32 by the City of Tallahassee and the 

USGS. To collect the composite sample, which represents 

the quality of water produced by the entire open interval 

of the test well under pumping conditions, a 6-in.-diameter 

submersible pump was set at 301 ft near the bottom of the well 

casing. A sampling port was installed on the side of the pump 

discharge pipe. Sample water passed through the port into 

Teflon tubing attached to a sampling chamber that protected 

the samples from atmospheric contamination during collection. The well was purged three volumes and, during the third 

volume, water chemistry was monitored for stability using 

a calibrated multiparameter water-quality instrument. Field 

readings consisted of specific conductance, pH, temperature, 

and dissolved oxygen measurements. On December 2, after 

USGS stabilization criteria were met, both the USGS and the 

City of Tallahassee collected a composite water-quality sample 

from the open interval of the test well (300–440 ft bls) while 

pumping at approximately 550 gallons per minute (gal/min).

Discrete depth-dependent water samples were collected 

within the open interval of the test well from the highly 

permeable flow zones that were identified using geophysical 

logs. On December 3, before sampling began, the drill crew 

installed a 4-in.-diameter submersible pump in CW32 capable 

of pumping about 60 gal/min. The 60-gal/min flow rate was 

ideal for discrete sampling because it allowed CW32 to be 

purged in less than 1 hour between samples, but the flow rate 

was low enough that it was not likely to induce intra-borehole 

flow over the length of the open interval and cause mixing 

of waters from each highly permeable zone. Ideally, packers 

would have been used to isolate each zone during sample 

collection, but the cavernous borehole precluded their use. The 

huge volume of water contained in the cavernous borehole 

makes it highly unlikely that mixing occurred at a flow rate of 

60 gal/min. The same sample collection procedures described 

above for the composite sample were followed for each of 

the discrete depth-dependent samples. The test well was 

purged at least three volumes, and field parameter stability 

was re-established, each time the pump was moved to a new 

highly permeable zone. Purge volume calculations were based 

on borehole volume alone, since it was impossible to estimate 

the volume of water contained in the cavernous portion of the 

open interval.

Water samples collected by the City of Tallahassee were 

analyzed for major ions, nutrients, dissolved organic carbon, 

trace elements, and volatile organic compounds by the City of 

Tallahassee Water-Quality Laboratory. The USGS collected 

water samples following methods described in the USGS 

National Field Manual for the Collection of Water-Quality 

Data (U.S. Geological Survey, variously dated). Water 

samples collected by the USGS were analyzed for tracers 

(including isotopes of oxygen, hydrogen, and strontium) and 

age-dating indicators (including tritium and sulfur hexafluoride). The samples collected by the USGS were analyzed 

at USGS research laboratories, including the Stable Isotope 

and Groundwater Dating Laboratories in Reston, Virginia, 

and the Tritium and Radiogenic Isotope Laboratories in 

Menlo Park, California. 

Source Tracers 

The stable isotopes of hydrogen (2H/H) and oxygen 

(18O/16O) are naturally occurring tracers that were used to 

determine whether the high-flow zones in CW32 produce 

water from similar sources. Rainfall around the world contains 

a consistent ratio of 18O/16O and 2H/H known as the “global 

meteoric water line” (GMWL; Craig, 1961). Isotope data are 

presented using delta (δ) notation, expressed as the ratio of the 

heavy to the light isotope, normalized to a standard:

δsample=1,000[(Rsample /Rstandard )–1] (1)

where Rsample and Rstandard are the ratio of the heavy to the light 

isotope in the sample and in the standard, respectively. Delta 

hydrogen-2 (δ2H, deuterium) and delta oxygen-18 (δ18O) 

are reported relative to Vienna Standard Mean Ocean Water 

(Coplen and others, 1991). Results are reported in units of 

per mil (parts per thousand).
4 Depth-Dependent Groundwater Quality Sampling at City of Tallahassee Test Well 32, Leon County, Florida, 2013

The isotopic variability along the GMWL depends on 

several factors, including the origin of storms, rainfall intensity and amount, and atmospheric temperature (Dansgaard, 

1964). After rainfall, water at land surface undergoes varying 

degrees of isotopic fractionation because of evaporation 

and other kinetic processes. Fractionation imparts a unique 

isotopic signature on water molecules that is maintained after 

recharge and allows groundwater to be traced to its source.

Strontium isotopes (87Sr/ 86Sr) are another natural tracer 

that was used to determine the geologic formation from which 

each highly permeable zone in CW32 yields water. Each 

formation has a unique range of isotopic ratios that correspond 

to the 87Sr/ 86Sr composition of the shallow seas in which the 

deposition of aquifer materials occurred. Groundwater reflects 

the same isotopic ratio as its host rock, so the source formation 

can be determined by comparing the 87Sr/ 86Sr ratio of a water 

sample with the known ratios of each aquifer formation (Katz 

and Bullen, 1996).

Age Dating

Sulfur hexafluoride (SF6) and tritium (3H) data were used 

to determine the similarity in age of the water from each of 

the highly permeable zones in CW32, thereby determining 

the connectedness of the flow system. Groundwater “age” 

is defined as the length of time a parcel of water has been 

isolated from the atmosphere, typically when water recharges 

to the groundwater system. Sulfur hexafluoride is a known 

greenhouse gas and is primarily used as an electrical insulator 

in high-voltage switches (Busenberg and Plummer, 2000). The 

atmospheric concentration of SF6 has steadily increased since 

the 1960s, which makes it useful for dating groundwater that 

has recharged since that time. Atmospheric moisture carries 

SF6 from the atmosphere to the surface, where it enters the 

groundwater system. A sample of water withdrawn from the 

groundwater flow system can then be dated by comparing the 

concentration of SF6 in the sample with known atmospheric 

concentrations over time (Busenberg and Plummer, 2000).

Tritium (3H) is a radioactive isotope of hydrogen with 

a half-life of 12.43 years. Tritium occurs naturally in the 

Earth’s atmosphere, but levels rose sharply after hydrogen 

bomb testing began in the 1950s. The concentration of 3H 

has steadily declined since 1963, when above-ground testing 

was banned, and now approaches natural background levels. 

Rainfall typically contains the same concentration of 3H 

as the atmosphere in which the rainfall originated, and this 

concentration is maintained as it recharges the groundwater. 

Radioactive decay, however, prevents 3H from being used, 

in the absence of other properties needed for corrections, to 

date groundwater in the way SF6 is used. Tritium by itself is 

useful for qualifying whether groundwater contains a mixture 

of young and old “pre-bomb” water. Water containing no 

measureable 3H has been isolated from the atmosphere since 

at least the mid-1950s (Ingraham, 1998).

Results

Three zones of high permeability were identified in the 

CW32 borehole. Water-quality, tracer, and age-dating results 

indicate that the open interval of CW32 produces water 

of consistent and high quality throughout its length. The 

cavernous nature of the open interval makes it likely that the 

highly permeable zones are interconnected in the aquifer by 

secondary porosity features.

Borehole-Geophysical Data

Borehole-geophysical data collection was completed 

in CW32 during December 2–3, 2013, to delineate zones of 

high permeability and support collection of depth-dependent 

groundwater samples. OBI and ABI data indicated that below 

343 ft bls the lithology appeared to be limestone with several 

large interconnected cavities. On the basis of caliper data, the 

cavities exceed 26 in. in diameter, the maximum diameter to 

which the caliper arms can extend. One such cavity is 15 ft 

long and is located 350 to 365 ft bls (appendixes 1 and 2). 

On the basis of borehole image and caliper data, the borehole 

wall above 343 ft bls also appears to be limestone containing 

strata of varying porosity. In the interval above 343 ft bls, the 

caliper remained fully engaged with the borehole wall and the 

diameter was consistently less than 14 in. 

Borehole fluid, natural gamma, and electromagneticinduction data indicated results similar to the image logs, 

whereby there was a noticeable difference in the data above 

and below 343 ft (appendixes 1 and 2). Minor changes in fluid 

properties, a decrease in fluid temperature, and an increase in 

specific conductance were observed at about 305 and 390 ft 

bls. Natural gamma and EM induction data had higher values 

and a greater range that varied with borehole diameter, indicating that the lithology above and below 343 ft bls may be 

similar and that the differences could be the result of changes 

in borehole diameter. In the cavernous section below 343 ft, 

the data from the full waveform sonic tool were unreliable 

due to borehole diameter, except where the borehole wall was 

close to the drilled diameter. 

Under ambient flow conditions prior to pumping from 

the well, the SFM was logged both up and down the well bore, 

and no observable flow was detected. The lack of measurable 

flow is substantiated by the change in fluid properties with 

depth, especially the change in fluid conductance. The change 

in conductance with depth is an indication that no flow or 

mixing occurs over the length of the open-hole section under 

ambient conditions. 

Under pumping conditions, most of the inflow from 

CW32 (as indicated by a change in counts per second of the 

SFM) originated from between 339 and 350 ft bls. This zone 

accounted for about 38 percent of the total well discharge. 

This zone of inflow may extend deeper to 365 ft bls, but flow 

within the cavernous zone cannot be measured with accuracy 
Results 5

(appendixes 1 and 2). Smaller zones of inflow were noted 

by the SFM data in the intervals 307–311 ft and 387–390 ft 

bls, with each accounting for about 25 percent of the total 

discharge. The pumped flow data were collected after an electrical submersible pump was installed in CW32. No testing 

was performed until after the rate of discharge stabilized. An 

inline flowmeter was installed on the pump discharge line, 

but it did not appear to operate properly, perhaps because of 

cavitation within the line. The flow rate was estimated to be 

about 550 gal/min using a bucket and stopwatch. Attempts 

were made to run the pump at varying speeds, but there was 

no apparent change in the rate of discharge. The lack of 

apparent change was supported by different logging runs of 

the SFM, which indicated no difference in flow velocity as the 

pump speed was varied. 

Approximately 3 hours after the pump was stopped 

and removed from CW32, the WQT was run a second time 

to determine how quickly fluid properties return to ambient 

pre-pumping conditions. Higher fluid conductance values were 

logged in the section above 343 ft bls (316 microsiemens per 

centimeter (μS/cm) at 25 degrees Celsius versus 260 μS/cm 

under ambient conditions) than were recorded prior to the 

test (appendix 1). These data are an indication that the 

550 gal/min pump rate was sufficient to induce flow over the 

entire open interval of the test well. Minor changes in other 

borehole fluid properties, including temperature, pH, percent 

oxygen, and redox potential, were similar to the pre-pumping 

ambient conditions. 

Following analyses of all the borehole-geophysical 

data, three highly permeable zones were delineated within 

the logged open interval of CW32 at 307–311 ft, 339–350 ft, 

and 387–390 ft. Pump placement was limited by the 20-ft 

length of each section of drop pipe attached to the pump 

and by the length of the pump housing itself, so it was not 

always possible to place the pump intake exactly adjacent to 

a permeable zone. Therefore, discrete depth-dependent water 

samples were collected at 301, 343, and 385 ft bls. 

Water-Quality Data

Water-quality data, including major ions, nutrients, trace 

elements, and volatile organic compounds, were consistent 

between samples with the exceptions of lead, copper, zinc, 

and toluene (tables 1, 2). Stiff diagrams, prepared using major 

ion data from each of the four samples, are nearly identical 

in shape (fig. 2). A Stiff diagram graphically displays the 

proportion of major ions in individual samples in a way that 

allows rapid visual comparison of the data on the basis of 

shape. The similarity of the major ion composition of the four 

samples indicates that the water has followed similar flow 

paths through the aquifer system.

Concentrations of lead, copper, zinc, and toluene tended 

to increase with depth, but were below established drinking 

water criteria (U.S. Environmental Protection Agency, 2014). 

It is unlikely that these constituents are present in the aquifer 

at the concentrations found in the samples. The samples 

likely were contaminated as the water passed through the 

drop pipe that supported the submersible pump. The pipe 

joints were coated with thread lubricant (pipe dope) at the 

connections, and all four of these substances are common 

ingredients in this type of lubricant. The galvanized finish on 

the pipes also may have contributed to sample contamination, 

especially with zinc, as it is the primary ingredient used in the 

galvanization process.

Source Tracers 

Results for δ18O and δ2H tracer data indicate that all four 

water samples originated from a similar source that is slightly 

enriched in isotopic composition compared to meteoric waters. 

Values for δ18O ranged from –2.69 to –2.78 per mil, whereas 

δ2H values ranged from –12.9 to –15 per mil (fig. 3). Water 

produced by CW32 contains a fraction of water recharged 

from a standing surface-water source, on the basis of the 

slightly enriched isotopic signature found in all of the samples 

collected from the test well. The composite sample was the 

most enriched of all the samples, likely because this sample 

contained a relatively large fraction of water recharged 

from standing surface-water sources. The higher pump rate 

(550 gal/min) used during the collection of the composite 

sample likely drew water in from shallower sources. The 

surface water could originate through a short-circuit pathway 

such as a sinkhole. Several sinkhole lakes and ponds are 

upgradient from CW32, including Lake Iamonia in the 

Summerbrooke Subdivision, and could be a source of this 

surface water. Alternatively, surface water could be drawn 

into the aquifer as a consequence of aquifer pumping at a 

point distant from the test well. This finding is consistent with 

results inferred for a site near this location and reported by 

Katz and others (1997), who determined that several City of 

Tallahassee production wells pumped groundwater mixed with 

a small percentage of surface water even though the wells 

were greater than about 200 ft deep.

Results for 87Sr/ 86Sr analyses indicate that water from all 

four samples followed a similar flow path through the Upper 

Floridan aquifer. Katz and Bullen (1996) analyzed rock cores 

collected from the Upper Floridan aquifer and determined 

that 87Sr/ 86Sr ratios for Eocene-aged Ocala Limestone vary 

from 0.7081 to 0.7085 per mil. Isotopic ratios in the four 

water samples collected at CW32 ranged from 0.70824 to 

0.70829 per mil, indicating that the Ocala Formation within 

the Upper Floridan aquifer is clearly the primary source of 

water to the test well (fig. 4). This result is expected because 

the open interval in the test well likely spans all or most of the 

Ocala Formation according to published estimates of aquifer 

depth and thickness (Miller, 1986).
6 Depth-Dependent Groundwater Quality Sampling at City of Tallahassee Test Well 32, Leon County, Florida, 2013

Table 1. Major ions, nutrients, and trace elements in groundwater samples collected at the City of Tallahassee test well 32, 

(U.S. Geological Survey site 303408084152300 COT 440 ft UFA Test Well at Tallahassee, FL), December 2013.

[ft, feet; EST, Eastern Standard Time; CaCO3, calcium carbonate; μg/L, micrograms per liter; <, less than; mg/L, milligrams per liter; CU, color units; ND, 

no data; N, nitrogen; TON, threshold odor number; NOD, no odor detected; °C, degrees Celsius; Ca, calcium; NTU, nephelometric turbidity units]

Sample description Composite sample

from 300 to 440 ft

Depth-dependent 

sample at 301 ft

Depth-dependent 

sample at 343 ft

Depth-dependent 

sample at 385 ft

Date (EST) 12/3/2013 12/4/2014 12/4/2013 12/4/2013

Time 1600 1500 1400 1200

Alkalinity, mg/L CaCO3 ND 176 175 175

Antimony, total, μg/L <2.49 <2.49 <2.49 <2.49

Arsenic, total, μg/L <1.39 <1.39 <1.39 <1.39

Cadmium, total <0.09 <0.09 <0.09 <0.09

Chloride, mg/L ND 3.8 3.9 3.9

Color, CU ND <1 <1 <1

pH ND 7.7 7.7 7.7

Fluoride, mg/L ND <0.20 <0.19 <0.19

Chlorine, free available, mg/L <0.0 <0.0 <0.0 <0.0

Lead, total, μg/L <1.32 <1.32 1.87 3.32

Mercury, total, μg/L <0.42 <0.42 <0.42 <0.42

Aluminum, total, μg/L <2.68 <2.68 <2.68 <2.68

Barium, total, μg/L 4.93 4.88 4.91 4.82

Beryllium, total, μg/L <0.15 <0.15 <0.15 <0.15

Calcium, total, mg/L 47.0 47.9 47.9 47.0

Chromium, total, μg/L 2.43 2.21 2.31 2.05

Cobalt, total, μg/L <0.31 <0.31 <0.31 <0.31

Copper, total, μg/L <0.36 <0.36 2.05 3.56

Iron, total, μg/L 11.5 9.21 21.4 5.90

Magnesium, total, mg/L 14.2 13.8 13.2 14.4

Manganese, total, μg/L <1.88 <1.88 <1.88 <1.88

Molybdenum, total, μg/L <1.17 <1.17 <1.17 <1.17

Nickel, total, μg/L <1.17 <1.17 <1.17 <1.17

Strontium, total, μg/L 84.1 82.5 81.7 81.9

Vanadium, total, μg/L 1.85 1.57 1.69 1.56

Zinc, total, μg/L 5.81 116 148 172

Nitrogen,nitrate by difference, mg/L as N ND 0.230 0.240 0.210

Nitrogen, nitrite, mg/L as N ND <0.001 <0.001 <0.001

Nitrogen, nitrite plus nitrate, mg/L as N ND 0.230 0.240 0.210

Odor, threshold, TON ND NOD NOD NOD

Potassium, total, mg/L 0.50 0.39 0.42 0.34

Selenium, total, μg/L <0.84 <0.84 <0.84 <0.84

Silica, dissolved, mg/L ND 15.86 16 15.97

Sodium, total, mg/L 2.52 2.48 2.51 2.45

Sulfate, total, mg/L ND 1.8 1.8 1.9

Residue, total filterable @ 180 °C, mg/L ND 202 198 197

Hardness, mg/L as Ca ND 181 177 183

Carbon, total organic, mg/L ND 0.4 0.4 0.4

Turbidity, NTU ND 0.19 0.18 0.10
Results 7

 Composite

(300–440 feet) 

301 feet

343 feet

385 feet

Cations

meq/l

Anions

5.0

0.0

5.0

Cl

HCO3+CO3

Mg SO4

Ca

Na+K

Cl

HCO3+CO3

Mg SO4

Ca

Na+K

Cl

HCO3+CO3

Mg SO4

Ca

Na+K

Cl

HCO3+CO3

Mg SO4

Ca

Na+K

Depth dependent

Figure 2. Stiff diagrams comparing the major ion concentrations of four water samples collected from City of Tallahassee 

test well 32, December 2013.

Figure 2. Stiff diagrams comparing the major ion concentrations of four water samples collected at the City of 

Tallahassee test well 32, December 2013.

Composite at 550 gallons 

per minute (300–440 feet)

385 feet at 60 gallons 

343 feet at per minute

60 gallons 

per minute

301 feet at 60 gallons 

per minute

–20

–19

–18

–17

–16

–15

–14

–13

–12

–11

–10

–4.0 –3.8 –3.6 –3.4 –3.2 –3.0 –2.8 –2.6 –2.4 –2.2 –2.0

Delta deuterium, per mil, VSMOW

Delta oxygen–18, per mil, VSMOW

Global meteoric water line

(Craig, 1961)

Evaporation trend line

Figure 3. Content of delta deuterium and delta oxygen-18 in water samples collected from the City of Tallahassee 

test well 32 relative to the global meteoric water line in December 2013.
8 Depth-Dependent Groundwater Quality Sampling at City of Tallahassee Test Well 32, Leon County, Florida, 2013

Table 2. Volatile organic compounds in groundwater samples collected from City of Tallahasse test well 32 

(U.S. Geological Survey site 303408084152300 COT 440 ft UFA Test Well at Tallahassee, FL), December 2013.—Continued

[EST, Eastern Standard Time]

Sample

description

Concentration of organic compounds, 

in micrograms per liter

Composite sample 

from 300 to 440 feet

Depth-dependent 

sample at 301 feet

Depth-dependent 

sample at 343 feet

Depth-dependent

sample at 385 feet

Date 12/3/2013 12/4/2013 12/4/2013 12/4/2013

Time (EST) 1600 1500 1400 1200

1,1,1,2-Tetrachloroethane <0.030 <0.030 <0.030 <0.030

1,1,1-Trichloroethane <0.030 <0.030 <0.030 <0.030

1,1,2,2-Tetrachloroethane <0.040 <0.040 <0.040 <0.040

1,1,2-Trichloroethane <0.030 <0.030 <0.030 <0.030

1,1-Dichloroethane <0.030 <0.030 <0.030 <0.030

1,1-Dichloroethylene <0.040 <0.040 <0.040 <0.040

1,1-Dichloropropene <0.030 <0.030 <0.030 <0.030

1,2,3-Trichlorobenzene <0.060 <0.060 <0.060 <0.060

1,2,3-Trichloropropane <0.150 <0.150 <0.150 <0.150

1,2,4-Trichlorobenzene <0.040 <0.040 <0.040 <0.040

1,2,4-Trimethylbenzene <0.030 <0.030 <0.030 <0.030

1,2-Dibromo-3-chloropropane <0.020 <0.020 <0.020 <0.020

1,2-Dibromoethane <0.070 <0.070 <0.070 <0.070

1,2-Dichlorobenzene <0.030 <0.030 <0.030 <0.030

1,2-Dichloroethane <0.030 <0.030 <0.030 <0.030

1,2-Dichloropropane <0.030 <0.030 <0.030 <0.030

1,3,5-Trimethylbenzene <0.030 <0.030 <0.030 <0.030

1,3-Dichlorobenzene <0.040 <0.040 <0.040 <0.040

1,3-Dichloropropane <0.030 <0.030 <0.030 <0.030

1,4-Dichlorobenzene <0.040 <0.040 <0.040 <0.040

2,2-Dichloropropane <0.040 <0.040 <0.040 <0.040

2-Chlorotoluene <0.020 <0.020 <0.020 <0.020

4-Chlorotoluene <0.030 <0.030 <0.030 <0.030

Benzene <0.040 <0.040 <0.040 <0.040

Bromobenzene <0.030 <0.030 <0.030 <0.030

Bromochloromethane <0.040 <0.040 <0.040 <0.040

Bromodichloromethane <0.040 <0.040 <0.040 <0.040

Bromoform <0.080 <0.080 <0.080 <0.080

Bromomethane <0.100 <0.100 <0.100 <0.100

Carbon tetrachloride <0.030 <0.030 <0.030 <0.030

Chlorobenzene <0.030 <0.030 <0.030 <0.030

Chloroethane <0.050 <0.050 <0.050 <0.050

Chloroform <0.100 <0.100 <0.100 <0.100

Chloromethane <0.080 <0.080 <0.080 <0.080

cis-1,2-Dichloroethylene <0.030 <0.030 <0.030 <0.030

cis-1,3-Dichloropropylene <0.030 <0.030 <0.030 <0.030

Dibromochloromethane <0.040 <0.040 <0.040 <0.040
Results 9

Table 2. Volatile organic compounds in groundwater samples collected from City of Tallahasse test well 32 

(U.S. Geological Survey site 303408084152300 COT 440 ft UFA Test Well at Tallahassee, FL), December 2013.—Continued

[EST, Eastern Standard Time]

Sample

description

Concentration of organic compounds, 

in micrograms per liter

Composite sample 

from 300 to 440 feet

Depth-dependent 

sample at 301 feet

Depth-dependent 

sample at 343 feet

Depth-dependent

sample at 385 feet

Dibromomethane <0.030 <0.030 <0.030 <0.030

Dichlorodifluoromethane <0.030 <0.030 <0.030 <0.030

Ethylbenzene <0.030 <0.030 <0.030 <0.030

Hexachlorobutadiene <0.040 <0.040 <0.040 <0.040

Isopropylbenzene <0.020 <0.020 <0.020 <0.020

Methylenechloride <0.030 <0.030 <0.030 <0.030

MTBE <0.050 <0.050 <0.050 <0.050

Naphthalene <0.040 <0.040 <0.040 <0.040

n-Butylbenzene <0.020 <0.020 <0.020 <0.020

n-Propylbenzene <0.030 <0.030 <0.030 <0.030

p-Isopropyltoluene <0.030 <0.030 <0.030 <0.030

sec-Butylbenzene <0.020 <0.020 <0.020 <0.020

Styrene <0.040 <0.040 <0.040 <0.040

t-Butylbenzene <0.030 <0.030 <0.030 <0.030

Tetrachloroethylene <0.040 <0.040 <0.040 <0.040

Toluene <0.030 0.490 1.240 1.740

Total Xylenes <0.050 <0.050 <0.050 <0.050

trans-1,2-Dichloroethylene <0.030 <0.030 <0.030 <0.030

trans-1,3-Dichloropropylene <0.040 <0.040 <0.040 <0.040

Trichloroethylene <0.030 <0.030 <0.030 <0.030

Trichlorofluoromethane <0.030 <0.030 <0.030 <0.030

Vinyl chloride <0.030 <0.030 <0.030 <0.030
10 Depth-Dependent Groundwater Quality Sampling at City of Tallahassee Test Well 32, Leon County, Florida, 2013

Age Dating

SF6 data indicate that water produced by the CW32 

open interval recharged to the groundwater system from 

26.2 to 29.4 years ago. The composite sample collected 

at 550 gal/min yielded the youngest date—26.2 years. 

The depth-dependent samples ranged in age from 

27.4 to 29.4 years; the samples from the greatest depth had 

the greatest ages (table 3). The piston-flow model used to 

relate SF6 concentration to age assumes that a parcel of water 

moves through a flow system with no mixing, but that is an 

unlikely scenario in a karst system. Therefore, the modeled 

ages most likely represent an average age for the mixture of 

waters produced by the well. The composite sample likely 

contains a relatively larger fraction of slightly younger water 

than from the discrete zones because the higher pump rate 

used during sample collection may have pulled younger water 

down from the overlying low permeability Hawthorn Group. 

This conclusion is supported by the 87Sr/ 86Sr data because the 

isotopic ratio of the composite sample was slightly higher 

than that of the discrete samples (fig. 4). The Hawthorn 

Group is known to have higher 87Sr/ 86Sr ratios than the Ocala 

Formation (Brewster-Wingard and others, 1997).

Tritium data corroborate the groundwater ages 

determined by SF6 dating and verify that most of the water 

produced by CW32 recharged to the groundwater system after 

hydrogen-bomb testing began in 1952. Tritium concentrations 

in the samples varied from 1.15 to 1.4 tritium units (TU; 

table 3). When the CW32 data are corrected for radioactive 

decay, using the range of recharge ages calculated from 

SF6 data, the concentration of 3H ranged from 5.0 to 7.0 TU at 

the time of recharge. The average 3H concentration in rainfall 

at Ocala, Fla., was 9.9 TU in 1984 and decreased to 4.7 TU by 

1988 (International Atomic Energy Agency, 2014); therefore, 

3

H data measured in this study support the calculated dates for 

SF6 recharge. Water recharged prior to 1952 would contain 3H 

at levels well below the 0.3 TU reporting limit of the analysis 

because the half-life of 3H is 12.43 years. CW32 likely 

produces little or no water recharged prior to 1952.

0.7080

0.7081

0.7082

0.7083

0.7084

0.7085

0.7086

0.0110 0.0115 0.0120 0.0125 0.0130 0.0135 0.0140

87Strontium/ 86Strontium

Strontium concentration inverse, in micrograms per liter

Figure 4. Relation between strontium isotope ratios and the inverse of the strontium concentration in water samples 

collected from City of Tallahassee test well 32, December 2013.

Composite

(300–440 feet) 385 feet

343 feet 301 feet

Ocala Limestone

Figure 4. Relation between strontium isotope ratios and the inverse of the strontium concentration in water samples 

collected at the City of Tallahassee test well 32, December 2013.
Summary and Conclusions 11

Table 3. Field measurements, tracer, and age data collected from City of Tallahassee test well 32, (U.S. Geological Survey site 

303408084152300 COT 440 ft UFA Test Well at Tallahassee, FL), December 2013.

[EST, Eastern Standard Time; ft blw LSD, feet below land surface datum; gal/min, gallons per minute; min, minutes; °C, degrees Celsius; μS/cm, microsiemens per centimeter at 25 degrees Celsius; mg/L, milligrams per liter; NTRU, nephelometric turbidity ratio units; CaCO3, calcium carbonate; avg, average; 

fg/kg, femtograms per kilogram; std, standard; δ18O, oxygen-18 to oxygen-16 ratio; δ2H, hydrogen-2 to hydrogen ratio, reported relative to Vienna Standard 

Mean Ocean Water (VSMOW); 3H, tritium; TU, tritium units; 87Sr/ 86Sr, strontium-87 to strontium-86 ratio]

Sample description Composite sample

from 300 to 440 feet

Depth-dependent

sample at 301 feet

Depth-dependent 

sample at 343 feet

Depth-dependent

sample at 385 feet

Date 12/3/2013 12/4/2013 12/4/2013 12/4/2013

Sample start time (EST) 1600 1500 1400 1200

Water level, ft blw LSD 194 194 194 194

Flow rate, gal/min 550 60 60 60

Pumping period, min 55 24 28 60

Sampling depth, ft blw LSD 300 301 343 385

Water temperature, °C 20.9 20.9 21.4 21.3

Specific conductance (field), μS/cm 330 340 339 344

pH (field) 7.4 7.4 7.4 7.5

Dissolved oxygen (field), mg/L 3.9 3.4 0.3 2.8

Turbidity (field), NTRU 1 0.4 0.3 0.3

Alkalinity (field), mg/L as CaCO3 176 185 183 184

Bicarbonate (field), mg/L 214 225 222 224

Carbonate (field), mg/L 0.4 0.3 0.3 0.3

Sulfur hexafluoride (SF6 ) (avg), fg/kg 191.5 152.5 146 117.5

Sulfur hexafluoride (SF6 ) piston flow 

model age (avg), years

26.2 27.4 27.7 29.4

Sulfur hexafluoride (SF6 ) piston flow 

model recharge year (avg)

1988 1986.5 1986 1984.5

δ18O, VSMOW –2.69 –2.7 –2.72 –2.78

δ2H, VSMOW –12.9 –13.2 –15 –13.6

3

H, TU 1.15 1.40 1.27 1.34

87Sr/ 86Sr 0.708 0.708 0.708 0.708

Summary and Conclusions 

The U.S. Geological Survey, in cooperation with the 

City of Tallahassee, completed a study to delineate highpermeability zones in the Upper Florida aquifer that have 

substantial rates of groundwater flow and to characterize 

changes in water quality with depth in the borehole of test 

well CW32 (CW32). Borehole-geophysical data were used 

to delineate high-permeability zones in the open interval of 

CW32. Water-chemistry data were used to determine the 

quality of water in the test well. Naturally occurring tracers 

were used to determine whether the high-flow zones in CW32 

produce water from similar sources, whereas age-dating 

was used to determine the approximate date of recharge 

(approximately 26 to 29 years ago) and if water from each of 

the highly permeable zones in CW32 is of similar age.

The open interval in CW32 produces water of similar 

(consistently) high quality throughout its length. Results of 

water-quality, tracer, and age-dating analyses indicate that the 

quality, source, flow path, and age of the water produced by 

CW32 are comparable in each of the three highly permeable 

zones identified using geophysical techniques. The cavernous 

nature of much of the open interval is an indication that the 

three productive zones are likely interconnected in the aquifer 

system by secondary porosity features. Changes to the CW32 

open interval, as constructed during testing, would likely 

provide little or no improvement in the high-quality water 

produced by the well.
12 Depth-Dependent Groundwater Quality Sampling at City of Tallahassee Test Well 32, Leon County, Florida, 2013

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Appendixes 13

Appendix 1. Comparison of borehole image, geophysical, 

water quality, flowmeter, and sonic logs showing evidence 

of three productive intervals in the City of Tallahassee 

test well 32 at Leon County, Florida, December 2013. Only 

the data collected in the open interval of the test well are 

displayed at 1 to 96 scale.

File can be downloaded from http://pubs.usgs.gov/of/2014/1255/.

Appendix 2. Comparison of borehole image, geophysical, 

water quality, flowmeter, and sonic logs showing evidence 

of three productive intervals in the City of Tallahassee test 

well 32 at Leon County, Florida, December 2013. The full 

geophysical log is displayed at 1 to 12 scale.

File can be downloaded from http://pubs.usgs.gov/of/2014/1255/.
Manuscript approved December 18, 2014

Prepared by the USGS Science Publishing Network

Raleigh Publishing Service Center

Edited by Kay P. Naugle

Illustrations by Kimberly A. Swidarski

Layout by Caryl J. Wipperfurth

For more information concerning this report, contact:

Director

U.S. Geological Survey

Caribbean-Florida Water Science Center

4446 Pet Lane, Suite 108

Lutz, FL 33559

(813) 498-5000

or visit our Web site at: 

http://fl.water.usgs.gov
McBride and Wacker—Depth-Dependent Groundwater Quality Sampling at City of Tallahassee Test Well 32, Leon County, Florida, 2013—Open-File Report 2014–1255

ISSN 2331-1258 (Online)

http://dx.doi.org/10.3133/ofr20141255