U.S. Department of the Interior

U.S. Geological Survey

Fact Sheet 2009–3062

July 2009

National Water-Quality Assessment, Transport of Anthropogenic and Natural Contaminants (TANC) to Public-Supply Wells

Assessing the Vulnerability of Public-Supply Wells to 

Contamination: Floridan Aquifer System Near Tampa, Florida

Printed on recycled paper

The aquifer systems near Tampa are composed of 

sand, clay, and carbonate rocks that were deposited 

primarily in a marine environment. The uppermost 

water-bearing unit consists of sand with varying 

amounts of silt and clay and is referred to as the 

“surficial aquifer system.” Water in this unit originates 

as rainfall or as lawn irrigation water that infiltrates 

to the water table. Yields of less than 5 gallons 

per minute, high iron content, and the potential for 

contamination make the surficial aquifer system 

unsuitable for use as a significant source of drinking 

water. Once infiltrating water reaches the water table, 

it typically moves downward and laterally through the 

surficial aquifer system until it reaches the underlying 

“intermediate confining unit,” a clay-rich layer that 

retards the downward movement of water into lower 

layers. The thickness of the intermediate confining 

unit is variable throughout the area, and it contains 

numerous breaches (called sinkholes) that form when 

the underlying limestone dissolves, causing the clay 

layer to collapse. Groundwater can move rapidly 

downward through these breaches, where it then enters the underlying Upper Floridan aquifer and resumes flow in a primarily 

lateral direction. The Upper Floridan aquifer is mostly limestone with some layers of dolomite and evaporites. Groundwater in the 

Upper Floridan aquifer may move slowly through the porous matrix or may travel at a relatively rapid rate through the many fractures 

and cavities that have been enlarged by dissolution of the limestone. The Upper Floridan aquifer is highly productive and is the 

principal source of freshwater in west-central Florida. The public-supply well selected for study is open to the Upper Floridan aquifer 

from 118 to 174 feet below land surface. This zone is tapped by many public-supply wells in the region, although the majority of 

wells have a longer open interval that extends to a greater depth than that of the selected well. A high-yielding, highly transmissive 

cavernous zone is intersected near the base of the open interval of the selected public-supply well. This cavernous zone is areally 

extensive and is noted on logs of many production and monitoring wells throughout the region.

Intermediate confining

unit

Surfical aquifer system 

(sand and clay)

Upper Floridan aquifer (limestone)

Cavernous zone

Not to scale

Recharge

Discharge

Water table Generalized groundwater flow

PublicUrban area supply well

Sinkhole

Sinkhole

The U.S. Geological Survey’s 

National Water-Quality Assessment 

(NAWQA) Program found, in studies 

from 1991 to 2001, low levels of mixtures of contaminants in groundwater 

near the water table in urban areas across 

the Nation. Contaminants were detected 

less frequently in deeper groundwater 

typically developed for public supply 

(Hamilton and others, 2004). The proximity of contaminant mixtures to underlying public water-supply sources, however, prompted the NAWQA Program to 

begin intensive studies in 2001 to assess 

the vulnerability of public-supply wells 

to contamination. Specifically, pathways 

and processes by which contaminants 

reach public-supply wells in nine aquifer 

systems across the country are being 

investigated. Scientists are studying the 

processes that occur below land surface—whereby contaminants are mobilized or attenuated—as well as investigating how human activities can affect the 

vulnerability of public-supply wells to 

contamination. 

This fact sheet highlights findings from the vulnerability study of a 

public-supply well in Temple Terrace, 

Florida (Katz and others, 2007; Crandall and others, in press), northeast 

of Tampa. The well typically produces 

water at the rate of 700 gallons per 

minute from the Upper Floridan aquifer.

In construction and operation, this well is 

similar to many other public-supply wells 

in the Tampa Bay region and throughout 

northern Florida. Historical problems in 

the public-supply wells of Temple Terrace include high concentrations of total 

coliform bacteria and fecal streptococci 

bacteria; however, the current requirement to disinfect all drinking water now 

protects users of publicly supplied water 

from health hazards associated with bacterial contamination. Samples collected 

from the selected public-supply well 

for the current study contained several 

types of chemical contaminants including nitrate, volatile organic compounds 

(VOCs), pesticides, uranium, radon-222, 
Study Design

A computer model of groundwater flow was constructed at a regional scale to estimate 

the “zone of contribution” to the selected public-supply well (Crandall, 2007). The zone 

of contribution is the three-dimensional volume of the aquifer material through which 

groundwater flows from the time it enters the groundwater system at the water table 

until it eventually discharges at the selected public-supply well. A network of monitoring 

wells was installed in or near the simulated zone of contribution to the well to understand 

groundwater flow and geochemistry along general directions of groundwater flow to the 

well. Water samples were collected from 28 monitoring wells and the supply well during 

2003 through 2005 and analyzed for naturally occurring contaminants (such as uranium 

and arsenic) and anthropogenic contaminants (such as volatile organic compounds, 

pesticides, and nutrients). Other water-quality parameters (such as major ions, agedating tracers, and selected stable isotopes) were analyzed for to improve understanding 

of sources of water and reactions affecting the chemical composition of groundwater. 

In addition, water samples were collected from the supply well at three depth intervals 

under nonpumping and pumping conditions to determine where water and contaminants 

enter the well and to assess the effect of pumping on water quality. Finally, a more 

detailed, local-scale computer model of groundwater flow to the selected public-supply 

well was constructed to estimate the traveltime of water particles entering the publicsupply well and to simulate long-term nitrate concentrations reaching the well (Crandall 

and others, in press). 

and arsenic. These contaminants were all 

detected in untreated water at concentrations less than established drinking-water 

standards, where such standards exist. 

Some of the contaminants detected 

in the selected public-supply well are 

anthropogenic (introduced by human 

activity); others occur naturally. Specifically, elevated concentrations of 

nitrate and the presence of pesticides are 

commonly linked with applications of 

fertilizer and pesticides to golf courses 

and lawns in urban and residential areas. 

VOCs are commonly used as solvents or 

gasoline components. Chloroform is a 

VOC that can be formed as a byproduct 

of chlorination. Publicly supplied water 

containing chloroform may reenter the 

groundwater system by way of leaking pipes or lawn irrigation. Uranium, 

radon-222, and arsenic occur naturally in 

the sediments and rocks that make up the 

aquifer system in the area. 

Overall, the study findings point to 

four primary factors that affect the movement and fate of contaminants and the 

vulnerability of the public-supply well in 

Temple Terrace, Florida: (1) groundwater age (how long ago water entered, or 

recharged, the aquifer); (2) short-circuiting of contaminated water through sinkholes; (3) natural geochemical processes 

within the aquifer; and (4) pumping 

stress. Study findings are intended to help 

water managers, drinking-water suppliers, and policy-makers to better understand how and why contamination of 

Floridan aquifer are mixtures of old 

water (recharged more than 60 years ago) 

and varying proportions of young water 

(recharged within the past 6 years). Estimated proportions of young water range 

from nearly zero for water from the deepest monitoring wells (screened 180 to 300 

feet below land surface) to 25 percent for 

the shallowest monitoring wells in the 

Upper Floridan aquifer. A few monitoring 

wells produce water from the cavernous 

zone (see text box describing aquifer); 

estimated proportions of young water for 

these wells is as great as 50 percent.

The selected public-supply well taps 

the Upper Floridan aquifer and the areally extensive cavernous zone. Age-tracer 

data suggest that a substantial fraction of

young water is produced by the publicsupply well. To better understand the 

blend of water of different ages (or “age 

distribution of water”) produced by the 

public-supply well, a computer-model 

simulation of groundwater flow was used 

to estimate the traveltime (or age) of 

water particles entering the public-supply 

well along the 56-foot length of the open 

interval. Modeled findings show that the 

simulated ages of particles reaching the 

public-supply well range from less than 1 

day to about 130 years (fig. 1). Roughly 

50 percent of the simulated flow to the 

well consists of water less than about 10 

years old. The youngest water enters the 

well through the cavernous zone near the 

base of the open interval in these simulations. The large percentage of young 

water produced by the public-supply well 

is noteworthy because young groundwater is vulnerable to contamination from 

human activities, whereas groundwater 

older than about 60 years recharged 

before the use of manmade chemicals 

became prevalent. 

Karst Features Serve as Short 

Circuits for Movement of 

Contaminants into the Public 

Supply Well 

The concentrations of anthropogenic 

compounds—such as nitrate (fig. 2) and 

VOCs—and the number of pesticide 

detections at the wellhead of the publicsupply well are intermediate between 

those detected in water from monitoring 

wells in the surficial aquifer system and 

the Upper Floridan aquifer. Similarly, the 

public-supply well produces water with 

concentrations of the natural contaminant 

public-supply wells occurs and whether 

water quality may improve or degrade. 

Additionally, study findings may be used 

to evaluate various pumping, resourcedevelopment, and land-management 

scenarios.

Well Vulnerability Results from 

Mixing of Very Recent Recharge 

with Older Water 

Groundwater age refers to the 

elapsed time since water entered or 

“recharged” an aquifer at the water table. 

Because water in an aquifer typically 

flows downward and laterally over time, 

it is expected that the very youngest 

water will be found near the water table 

and older water will be found at greater 

depths. Groundwater age can be estimated by measuring concentrations of 

modern age tracers (tritium, its radioactive decay product helium-3, and sulfur 

hexafluoride). Concentrations of tritium 

and sulfur hexaflouride increased in the 

atmosphere—and also in precipitation—

within the last five to six decades as a 

result of human activities. Thus, groundwater containing these tracers recharged 

the aquifer within the last 60 years. 

In the Temple Terrace area, agetracer data from monitoring wells tapping a surficial aquifer system indicate 

that most of the water in this aquifer 

recharged within the past 6 years. In 

contrast, water samples from monitoring wells in the underlying Upper 
Figure 1. A groundwater 

flow model was used to 

simulate the mixture of 

water of different ages 

produced by the selected 

public-supply well in Temple 

Terrace, Florida.

try than water entering the well from a 

shallower zone in the Upper Floridan 

aquifer during nonpumping conditions. 

Specifically, water from the cavernous zone had higher concentrations of

nitrate, radon-222, and trichloromethane 

(chloroform), as well as lower specific 

conductance, compared to water from 

the shallower depth interval sampled. 

To reach the public-supply well, water 

from the surficial aquifer system likely 

moves downward through breaches in 

the intermediate confining unit caused 

by sinkholes. The cavernous zone tapped 

at the bottom of the open interval of the 

selected public-supply well (and several 

other supply wells in the area) must be 

directly connected to one or more sinkholes and conduits. 

Natural Geochemical Processes 

in Aquifer Affect the Fate of 

Contaminants 

Geochemical conditions, such as 

oxygen content of groundwater, control 

whether specific contaminants are attenuated or mobilized in the subsurface. In 

the oxic conditions generally found in 

the surficial aquifer system (dissolved 

oxygen concentrations of at least 1 mg/L 

(milligram per liter)), the anthropogenic 

contaminant nitrate is stable and can be 

transported long distances. Thus, it is not 

surprising that elevated nitrate-N concentrations (maximum of 6.1 mg/L) were 

found in the monitoring wells completed 

in the surficial aquifer system. However, 

in the “anoxic” conditions (dissolved 

oxygen concentrations less than 0.5 

mg/L) typically present in monitoring 

wells of the intermediate confining unit 

and in the Upper Floridan aquifer, nitrate 

is naturally attenuated by a process 

known as denitrification, in which nitrate 

is converted to nitrogen gas. Because of 

this process, concentrations of nitrate in 

water from the Upper Floridan aquifer 

are generally low (less than the detection 

limit of 0.06 mg/L). 

In contrast to nitrate, the concentration of the natural contaminant radon-222 

is not influenced by the oxygen content 

of the aquifers but is instead controlled 

by the geochemical makeup of the aquifer matrix and the process of radioactive 

decay. Radon-222 is one of the products 

formed by radioactive decay of the parent 

material uranium-238. In turn, radon-222 

itself decays. Concentrations of radon 

were found to be highest in the surficial 

aquifer system and intermediate confining unit, where uranium-238 was more 

concentrated in the aquifer matrix. As 

Figure 2. Concentrations of nitrate in the selected public-supply well are intermediate 

between those found in the Upper Floridan aquifer and the surficial aquifer system.

SURFICIAL AQUIFER 

SYSTEM

PUBLIC-SUPPLY 

WELL

UPPER FLORIDAN 

AQUIFER

0

1

2

3

4

5

6

7

NITRATE CONCENTRATION, 

IN MILLIGRAMS PER LITER AS N 

Detection limit 

(0.06 milligram 

per liter)

Outlier 1

Upper whisker 2

1 Outliers are defined as values outside of the whisker span

2 The whiskers span thelargest and smallest values within 1.5 times the interquartilerange. 

75th 

percentile

Lower whisker

Median

25th 

percentile

.

radon-222 and stable isotopes of oxygen and hydrogen that are intermediate 

between the typical composition of water 

from the surficial aquifer system and that 

from the Upper Floridan aquifer. These 

findings suggest that the public-supply 

well produces a mixture of water from 

the two aquifers. Mixing calculations 

based on geochemical mass balance show 

that the water produced by the publicsupply well could consist of upwards 

of 50 percent water from the surficial 

aquifer mixed with water from the Upper 

Floridan aquifer. 

Water from the surficial aquifer 

system appears to reach the public-supply 

well through the cavernous zone near 

the base of the open interval. Samples of 

water entering the supply well from this 

zone had a distinctly different chemis0

4

8

12

16

0 20 40 60 80 +100

AGE , IN YEARS

PERCENTAGE OF WATER ENTERING WELL

10 30 50 70 90

“Water age” refers to the time elapsed since water entered the groundwater system 

as recharge at the water table. The peak at the left edge of the graph is very young 

water from the surficial aquifer system that flows rapidly to the well through karst 

features in the underlying Upper Floridan aquifer. This water combines with older 

water that slowly flows through the intermediate confining unit and Upper Floridan 

aquifer. (Graph modified from McMahon and others, 2008.)
groundwater travels downward into the 

Upper Floridan aquifer, away from the 

higher concentrations of parent material, radon-222 concentrations generally 

decrease as the radon decays—from 

median concentrations of 2,440 pCi/L 

(picocuries per liter) in the surficial aquifer system to 2,100 pCi/L in the intermediate confining unit and 645 pCi/L in 

the Upper Floridan aquifer. The effect 

on human health of ingesting water with 

these concentrations of radon is unclear, 

because the U.S. Environmental Protection Agency (USEPA) has not finalized 

regulations setting a Maximum Contaminant Level (MCL) for radon in drinking 

water. All sampled waters had radon 

concentrations that exceed the proposed 

MCL of 300 pCi/L but were well below a 

proposed alternative MCL of 4,000 pCi/L 

that factors in plans to address radon in 

indoor air (U.S. Environmental Protection Agency, 1999). 

If the intermediate confining unit 

were intact throughout the Tampa area, 

natural attenuation—through the processes of denitrification and radioactive 

decay— would lessen the concentrations of nitrate and radon-222 reaching 

the public-supply well. These processes 

are circumvented, however, by the karst 

features such as sinkholes that breach 

the intermediate confining unit and serve 

as fast pathways to the well. Instead 

of traveling slowly through the aquifer 

matrix, young water from the surficial 

aquifer system may be shunted directly 

into a conduit, where it spends relatively 

little time before being captured and discharged by the supply well. Thus, attenuating reactions such as denitrification and 

radioactive decay may not progress as 

far in the cavernous zone as elsewhere in 

the Upper Floridan aquifer. Differences 

in reaction progress among the different 

parts of the Upper Floridan aquifer are 

likely exacerbated when local pumping 

centers further accelerate the movement 

of water in the cavernous zone. Because 

water produced by the public-supply well 

is a mixture of relatively young, oxygenated water from the cavernous zone and 

older, anoxic water from other parts of 

the Upper Floridan aquifer, contaminants 

such as nitrate and radon in this and other 

supply wells intersecting the cavernous 

zone are somewhat diluted relative to 

waters of the surficial aquifer system. 

Pumping of Public-Supply Well 

Enhances Vulnerability to Arsenic 

Arsenic concentrations in samples 

from the public-supply wellhead remained 

at or below 3.5 mg/L (micrograms per 

liter) throughout the study—below the 

MCL of 10 mg/L. However, depth-dependent sampling while the public-supply 

well was being pumped showed that water 

entering the well from the cavernous 

zone near the base of the open interval 

contained arsenic and dissolved oxygen 

concentrations as high as 18.9 mg/L and 

1.0 mg/L, respectively. Surprisingly, during nonpumping conditions on the previous day, water from this same zone had a 

substantially lower arsenic concentration 

(4.2 mg/L) and a lower dissolved oxygen 

concentration (0.8 mg/L). This implies 

that movement of arsenic and dissolved 

oxygen to the public-supply well from 

the cavernous zone in the Upper Floridan 

aquifer is enhanced by pumping. 

The movement of oxygenated water 

during pumping may actually liberate 

arsenic from the aquifer matrix. Arthur 

and others (2002) noted that arsenic 

was mobilized from the matrix of the 

Upper Floridan aquifer when oxic water 

was injected into an aquifer storage and 

recovery well field in Tampa. When the 

water was recovered from the Upper 

Floridan aquifer, it contained arsenic at 

concentrations more than 5 times the 

initial concentration in either the injected 

water or the native groundwater. One 

possible explanation for the higher arsenic 

concentrations during pumping is that the 

higher oxygen content of the groundwater 

in the cavernous zone (than elsewhere in 

the Upper Floridan aquifer) causes dissolution of arsenic associated with pyrite. 

Pyrite is a naturally occurring mineral 

the Upper Floridan aquifer that is most 

abundant in high-porosity zones (Price 

and Pichler, 2006). 

Protection Effort Implications

Public-supply wells that intersect 

solution features may benefit from a 

secondary area of protection. Efforts to 

protect drinking-water supplies derived 

from groundwater generally rely on 

restricting activities within the area 

contributing recharge to a public-supply 

well. The State of Florida requires a primary protection zone within a radius of 

500 feet around the well, where activities 

such as construction of new landfills or 

new aboveground or underground hazardous waste storage tanks are prohibited 

and other new installations, such as new 

domestic wastewater treatment facilities, 

must meet stringent performance standards. Communities are encouraged (but 

are not required) to define a secondary 

area of protection around the wellhead by 

using methods such as a time-of-travel 

calculation (Florida Department of Environmental Protection, 2008) to delineate 

The sinkholes peppering this Florida forest are typical of karst topography that forms when 

underlying limestone partially dissolves. Other solution features (like the cavernous zone 

intersected by several wells in this study) may form along horizontal bedding planes and not 

be evident at land surface. (Photo by Ann Tihansky, U.S. Geological Survey.)
the aquifer volume through which water 

will travel to the well within a certain 

amount of time. Figure 3 shows the simulated area contributing recharge to the 

selected public-supply well and associated times for water particles recharging the water table to travel to the well. 

Although some simulated areas associated with rapid traveltimes (less than 1 

day to 6.4 years) are adjacent to the public-supply well, some are also more than 

2 miles from the wellhead, owing to rapid 

traveltimes along flow paths that follow 

solution features in the Upper Floridan 

aquifer. Figure 3 demonstrates that, for 

wells intersecting solution features in the 

Upper Floridan aquifer, restricting activities only within a 500-foot radius around 

the well may not provide adequate 

protection or time to react if incidents of 

unexpected contamination (such as spills 

or leakage from chemical storage tanks) 

do occur. Wells that intersect solution 

features may warrant additional study to 

provide information upon which a more 

effective secondary area of protection can 

be based. For example, the traveltimes 

shown in figure 3 were estimated by use 

of a groundwater-flow model that took 

into account sinkhole locations and preferential flow along fractures and solution 

features, as well as average withdrawals 

and the interval of the aquifer tapped by 

the 76 other public-supply, industrial, or 

agricultural wells in the modeled area. 

Similarity of public-supply well 

water to that of the surficial aquifer 

system is one measure of well vulnerability that could be used to prioritize 

protection efforts. Although water from 

the surficial aquifer system is recognized 

as being less desirable for use as drinking water than that of the Upper Floridan 

aquifer, in reality the surficial aquifer 

system probably makes up a notable proportion of the water produced by publicsupply wells in the Tampa area that are 

completed in the Upper Floridan aquifer 

and intersect a solution feature. Even 

though the open interval of the publicsupply well selected for study is more 

than 115 feet below land surface in an 

aquifer separated from the surficial aquifer system by a discontinuous confining 

layer, Katz and others (2007) determined 

that upwards of 50 percent of the water 

produced by the public-supply well may 

be very young water from the surficial 

aquifer system. 

All public-supply wells in the area 

may not have this same level of vulnerability—many of the public-supply 

wells in the region have lower pumping 

rates and have open intervals that extend 

Figure 3. The colored zones on the map show the simulated area contributing recharge to a supply well selected for investigation in Temple 

Terrace, Florida. These zones indicate where much of the water that eventually reaches the well likely enters the groundwater system at the 

water table. The exact location of these zones cannot be defined with certainty due to lack of data regarding the location and interconnectedness 

of subsurface solution features. However, the simulated, noncontiguous pattern of recharge is realistic given that karst features allow water from 

the surficial aquifer system to short-circuit the intermediate confining unit and much of the Upper Floridan aquifer and travel to the supply well in 

unexpected ways. Restricting land-use activities in a 500-foot radius around the well (the primary protection zone required by the State of Florida) 

protects only a small portion of the area contributing recharge that is associated with rapid traveltimes to the well.

deeper into the Upper Floridan aquifer 

than the well studied. Thus, such wells 

may draw in proportionally less water 

from the surficial aquifer system than the 

selected well, and what they do draw in 

may be diluted by a larger proportion of 

old water from deeper in the Upper Floridan aquifer that predates anthropogenic 

influences. One way to evaluate the level 

of vulnerability of various public-supply 

wells in the region would be to determine 

the degree to which the water quality in 

these wells reflects that of the surficial 

aquifer system as described by Katz and 

others (2007). Such knowledge could 

be used to help prioritize monitoring 

and land-use planning efforts to protect 

the most vulnerable wells (Landon and 

others, 2006). If the suggested analysis 

shows that deeper public-supply wells 

are less vulnerable, water suppliers could 

consider deepening the most vulnerable 

wells and constructing new wells with 

longer and deeper open intervals to maximize in-well dilution. 

Concentrations of contaminants 

produced by the public-supply well are 

strongly influenced by land-surface 

activities over the last decade. Nitrate 

concentrations in the public-supply well 

over time are a result of (1) the mixture of ages of water reaching the well 

DEPRESSIONS

RIVER, STREAMS,

AND WETLANDS

PATHLINES

PUBLIC-SUPPLY WELL

AREA CONTRIBUTING RECHARGE 

(TRAVELTIME, IN YEARS)

0 1 2 KILOMETERS

0 0.5 1 MILES

0.5 1.5

< 1 day to 6.4

>6.4 to 16

>16 to 26

>26 to 37

>37 to 50

>50 to 66

>66 to 127

Public-Supply Well 0 1 2 KILOMETERS

0 1 MILE

Hillsborough

River

Area

shown at left

City of

Temple

Terrace

Public- Supply Well

PRIMARY PROTECTION

ZONE
For more information on the Transport of Anthropogenic and Natural Contaminants to Supply Wells (TANC) topical study, see

http://oh.water.usgs.gov/tanc/NAWQATANC.htm

Contacts

Sandra Eberts, Team Leader

(614) 430-7740, 

smeberts@usgs.gov

Christy Crandall, Groundwater Flow 

Modeling

(850) 553-3674, 

crandall@usgs.gov

Brian Katz, Geochemistry

(850) 553-3671, 

bkatz@usgs.gov

References 

Arthur, J.D., Dabous, A.A., and Cowart, 

J.B., 2002, Mobilization of arsenic and 

other trace elements during aquifer storage and recovery, southwest 

Florida, in Aiken, G.R., and Kuniansky, E.L., eds., U.S. Geological Survey 

Artificial Recharge Workshop Proceedings: U.S. Geological Survey OpenFile Report 02–89, p. 47–50, accessed 

May 22, 2009, at http://water.usgs.gov/

ogw/pubs/ofr0289/ 

Crandall, C.A., 2007, Hydrogeologic 

setting and ground-water flow simulations of the Northern Tampa Bay 

Regional Study Area, Florida, section 

5 of Paschke, S.S. ed., Hydrogeologic settings and ground-water flow 

simulations for regional studies of the 

transport of anthropogenic and natural 

contaminants to public-supply wells—

Studies begun in 2001: U.S. Geological Survey Professional Paper 1737–A, 

sec. 5, accessed March 17, 2008, at 

http://pubs.usgs.gov/pp/2007/1737a

Crandall, C.A., Kauffman, L.J., Katz, 

B.G., Metz, P.A., McBride, W.S., and 

Berndt, M.P., in press, Simulations of 

groundwater flow and particle tracking analysis in the area contributing 

recharge to a public-supply well near 

Temple Terrace, Florida, 2002–05: U.S. 

Geological Survey Scientific Investigations Report 2008–5231, 53 p.

Florida Department of Environmental 

Protection, 2008, Wellhead protection: Accessed September 16, 2008, 

at http://www.dep.state.fl.us/water/

groundwater/wellhead.htm

Hamilton, P.A., Miller, T.L., and Myers, D.N., 

2004, Water quality in the Nation’s streams 

and aquifers—Overview of selected findings, 1991–2001: U.S. Geological Survey 

Circular 1265, 20 p. 

Katz, B.G., Crandall, C.A., Metz, P.A., 

McBride, W.S., and Berndt, M.P., 

2007, Chemical characteristics, water 

sources and pathways, and age distribution of ground water in the contributing area of a public-supply well 

near Tampa, Florida, 2002–05: U.S. 

Geological Survey Scientific Investigations Report 2007–5139, 85 p.

Landon, M.K., Eberts, S.M., Jurgens, 

B.C., Katz, B.G., Burow, K.R., 

Crandall, C.A., Brown, C.J., Starn, 

J.J., 2006, Knowledge of where and 

how contamination-susceptible water 

enters public-supply wells can be 

used to improve monitoring strategies 

and protection plans: Groundwater 

Protection Council Annual Forum 

2006, Sept. 30–Oct. 4, 2006, Miami 

Beach, Fla., 15 p., accessed October 7, 

2008, at http://oh.water.usgs.gov/tanc/

pubs/Landon_Eberts_CA-3521_gwpc_

paper_final_ss.pdf

McMahon, P.B., Böhlke, J.K., Kauffman, 

L.J., Kipp, K.L., Landon, M.K., Crandall, C.A., Burow, K.R., and Brown, 

C.J., 2008, Source and transport controls 

on the movement of nitrate to public supply wells in selected principal aquifers 

of the United States: Water Resources 

Research, v. 44, W04401, 17 p.

Price, R.E., and Pichler, Thomas, 2006, 

Abundance and mineralogical association of arsenic in the Suwannee 

Limestone (Florida)—Implications 

for arsenic release during water-rock 

interaction: Chemical Geology, v. 228, 

p. 44–56.

U.S. Environmental Protection Agency, 

1999, Proposed radon in drinking water 

rule: EPA 815–F–99–006, 6 p.

By Martha L. Jagucki, 

Brian G. Katz, Christy A. Crandall, 

and Sandra M. Eberts

combined with (2) changing nitrogen 

inputs at land surface over time (from 

sources such as application of inorganic 

fertilizer) and (3) denitrification rates 

in the intermediate confining unit and 

Upper Floridan aquifer. The effect of 

these three factors on long-term nitrate 

concentrations in the public-supply well 

was estimated by using the local-scale 

groundwater model. The simulated age 

distribution of water produced by the 

public-supply well (fig. 1) was used to 

determine the proportions of water that 

recharged in different years. Proportions of recharging water during a given 

year were then linked with the nitrate 

concentration in recharge during that 

year as estimated by McMahon and others (2008). Input nitrate concentrations 

were tracked as they moved through the 

geologic layers in the model. In the layers representing the oxic surficial aquifer 

system, nitrate concentrations were not 

adjusted. Once water particles entered 

the anoxic intermediate confining unit 

and Upper Floridan aquifer, probable 

minimum and maximum denitrification 

rates were applied to particles until they 

reached the public-supply well. The final 

calculated nitrate concentration at the 

public-supply well during a given year 

was the sum of nitrate concentrations of 

all water particles reaching the well that 

year, taking into account flow volumes 

represented by the particles. The calculated long-term nitrate concentrations in 

the selected public-supply well indicate 

a 1- to 10-year time lag between peak 

input concentrations in groundwater 

recharge and peak concentrations in 

the public-supply well. This short lag 

time reflects the large proportion of 

young water produced by the well and 

the strong influence that inputs at land 

surface within the last decade have on 

contaminant concentrations in water currently reaching the well. Future projections indicate that if fertilizer application 

rates in the urban area remain at current 

levels and land use remains fixed at the 

current proportion of 84 percent urban, 

then nitrate concentrations in the selected 

public-supply well will remain at about 

1 to 3 mg/L in the future.