Simulations of Groundwater Flow and 

Particle Tracking Analysis in the Area 

Contributing Recharge to a Public-Supply 

Well near Tampa, Florida, 

2002-05

By Christy A. Crandall, Leon J. Kauffman, Brian G. Katz, Patricia A. Metz, 

W. Scott McBride, and Marian P. Berndt

Prepared in cooperation with the

National Water-Quality Assessment Program Transport of Anthropogenic

and Natural Contaminants (TANC) to Public-Supply Wells

Scientific Investigations Report 2008–5231

U.S. Department of the Interior

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

KEN SALAZAR Secretary

U.S. Geological Survey

Suzette M. Kimball, Acting Director

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

For more information on the USGS—the Federal source for science about the Earth, its natural and living resources, 

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reproduce any copyrighted materials contained within this report.

Suggested citation:

Crandall, C.A., Kauffman, L.J., Katz, B.G., Metz, P.A., McBride, W.S., and Berndt, M.P., 2009, Simulations of 

groundwater flow and particle tracking analysis in the area contributing recharge to a public-supply well near 

Tampa, Florida, 2002-05, U.S. Geological Survey Scientific Investigations Report 2008-5231, 53 p.

978 1 4113 2548 7
iii

Foreword

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

information that helps to enhance and protect the overall quality of life and that facilitates effective 

management of water, biological, energy, and mineral resources (http://www.usgs.gov/). Information 

on the Nation’s water resources is critical to ensuring long-term availability of water that is safe for 

drinking and recreation and is suitable for industry, irrigation, and fish and wildlife. Population growth 

and increasing demands for water make the availability of that water, measured in terms of quantity and 

quality, even more essential to the long-term sustainability of our communities and ecosystems.

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

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

and policy (http://water.usgs.gov/nawqa). The NAWQA Program is designed to answer: What is the 

quality of our Nation’s streams and ground water? How are conditions changing over time? How do 

natural features and human activities affect the quality of streams and ground water, and where are 

those effects most pronounced? By combining information on water chemistry, physical characteristics, 

stream habitat, and aquatic life, the NAWQA Program aims to provide science-based insights for 

current and emerging water issues and priorities. From 1991 to 2001, the NAWQA Program completed 

interdisciplinary assessments and established a baseline understanding of water-quality conditions in 51 

of the Nation’s river basins and aquifers, referred to as Study Units (http://water.usgs.gov/nawqa/studyu.

html). 

National and regional assessments are ongoing in the second decade (2001–2012) of the NAWQA 

Program as 42 of the 51 Study Units are selectively reassessed. These assessments extend the findings 

in the Study Units by determining water-quality status and trends at sites that have been consistently 

monitored for more than a decade, and filling critical gaps in characterizing the quality of surface water 

and ground water. For example, increased emphasis has been placed on assessing the quality of source 

water and finished water associated with many of the Nation’s largest community water systems. During 

the second decade, NAWQA is addressing five national priority topics that build an understanding of 

how natural features and human activities affect water quality, and establish links between sources 

of contaminants, the transport of those contaminants through the hydrologic system, and the potential 

effects of contaminants on humans and aquatic ecosystems. Included are studies on the fate of 

agricultural chemicals, effects of urbanization on stream ecosystems, bioaccumulation of mercury in 

stream ecosystems, effects of nutrient enrichment on aquatic ecosystems, and transport of contaminants 

to public-supply wells. In addition, national syntheses of information on pesticides, volatile organic 

compounds (VOCs), nutrients, trace elements, and aquatic ecology are continuing. 

The USGS aims to disseminate credible, timely, and relevant science information to address practical and 

effective water-resource management and strategies that protect and restore water quality. We hope this 

NAWQA publication will provide you with insights and information to meet your needs, and will foster 

increased citizen awareness and involvement in the protection and restoration of our Nation’s waters. 

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

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

and conservation of our Nation’s water resources. The NAWQA Program, therefore, depends on advice 

and information from other agencies—Federal, State, regional, interstate, Tribal, and local—as well as 

nongovernmental organizations, industry, academia, and other stakeholder groups. Your assistance and 

suggestions are greatly appreciated.

Matthew C. Larsen

Associate Director for Water
iv

Acknowledgments

The authors thank the City of Temple Terrace, Florida, for their cooperative support, especially Woody 

Garcia, former Director of Public Works for the City of Temple Terrace. Thanks are extended to Dana 

Carver of the Recreation Department and to the Public Works Department for the City of Temple Terrace 

for their generosity, assistance, and cooperation. The authors thank Eric Dehaven and the geophysical 

technicians at the Southwest Florida Water Management District for their help in obtaining pumping data 

and geophysical support and data during depth sampling of the selected public-supply well. 

John Williams of the U.S. Geological Survey Office of Groundwater provided invaluable geophysical 

support and interpretation.
v

Contents

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

Acknowledgments......................................................................................................................................................................iv

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

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

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

Previous Studies.................................................................................................................................................................2

Description of Study Area..........................................................................................................................................................4

Climate ..................................................................................................................................................................................4

Population and Land Use..................................................................................................................................................6

Physiography.......................................................................................................................................................................6

Surface-Water and Groundwater Interactions ..........................................................................................................7

Hydrogeologic Setting.......................................................................................................................................................7

Geology .......................................................................................................................................................................7

Lithology......................................................................................................................................................................7

Groundwater Flow System.....................................................................................................................................9

Methods........................................................................................................................................................................................10

Design of Sampling Network and Data Collection and Analysis..........................................................................10

Development and Calibration of Local-Scale Groundwater Flow and Particle Tracking Model..................15

Groundwater Flow and Advective Tracking Simulation ...................................................................................................16

Model Uncertainties and Limitations...........................................................................................................................16

Model Geometry and Discretization............................................................................................................................17

Boundary Conditions and Model Stresses ................................................................................................................19

Description of Lateral Boundaries .....................................................................................................................19

Simulation of Recharge and Discharge............................................................................................................20

Pumping....................................................................................................................................................................20

Hydraulic and Other Estimated Parameters..............................................................................................................22

Hydraulic Parameters............................................................................................................................................26

Recharge Parameters...........................................................................................................................................28

Other Parameters...................................................................................................................................................28

Parameter Sensitivity ............................................................................................................................................28

Simulation of Particle Tracking...............................................................................................................................................29

Calibration....................................................................................................................................................................................30

Water Levels and Water-Level Gradients ..................................................................................................................30

Flow Patterns.....................................................................................................................................................................30

Water Budget....................................................................................................................................................................36

Particle Tracking...............................................................................................................................................................36

Age Tracer Concentrations..................................................................................................................................37

Simulated Particle Ages .......................................................................................................................................37

Comparison of Land Use in the Area Contributing Recharge and Occurrence of Selected Contaminants........39

Long-Term Concentrations of Nitrate in the Public-Supply Well....................................................................................41

Summary and Conclusions ......................................................................................................................................................45

References Cited........................................................................................................................................................................46

Appendix.................................................................................................................................................................................49-53
vi

Figures

1. Map showing regional extent of the Floridan aquifer system, aquifer confinement status, 

 major cities in Florida, and the TANC regional- and local-scale model boundaries in the 

 Tampa Bay area......................................................................................................................................................... 3

2. Map showing local-scale study area projected over the digital-orthographic-quarter 

 quadrangle showing City of Temple Terrace boundary, nearby cities, springs, streams, gaging 

 stations, and the selected public-supply well.................................................................................................... 5

3. Digital Elevation Model (DEM) of the local-scale study area with 20x exaggeration of 

 vertical profile of the local-scale study area, showing the remnant Wicomico Terrace, 

 numerous sinkholes projected from the DEM and the Northern Wetlands ............................................... 6

4. Geologic and hydrogeologic stratigraphic column............................................................................................ 8

5. Conceptual diagram of recharge, hydrogeology, and groundwater flow system dynamics for

 the local-scale study area..................................................................................................................................... 10

6. Diagram showing median measured water levels and simulated potentiometric 

 surfaces of the surficial aquifer system and Upper Floridan aquifer in the local-scale 

 study area, 2000 ....................................................................................................................................................... 11

7. Map showing locations of wells and test holes installed for this study and trace of

 hydrogeologic sections A-A’ and B-B’............................................................................................................... 12

8. Hydrogeologic sections across the study area ................................................................................................ 14

9. Graph showing median daily water levels from continuous water-level recorders in

 monitoring wells, stormwater retention ponds, and the Hillsborough River station, and

 various isolated measurements made in the local-scale study area, December 11, 2003

 to November 17, 2005.............................................................................................................................................. 16

10-14. Maps showing:

10. Local-scale model area in relation to the regional-scale modeled area............................................ 18

11. Distribution of recharge in the local-scale study area............................................................................ 21

12. River and drain cells in the local-scale model and study areas ........................................................... 22

13. Location of groundwater withdrawal wells and gaging stations in the local-scale

 study area .......................................................................................................................................................... 23

14. Distribution of horizontal hydraulic conductivity in model layers 1 to 3, representing

 the surficial aquifer system ........................................................................................................................... 27

15. Graph showing the composite scaled sensitivity of the parameters used to calibrate the

 groundwater flow and transport model in UCODE.......................................................................................... 29

16. Graph showing simulated and measured hydraulic heads, hydraulic head gradients, sulfur

 hexafluoride and tritium concentrations, and corresponding weighted/nonweighted

 simulated and measured values.......................................................................................................................... 32

17-19. Maps showing:

17. Simulated and measured hydraulic heads in the surficial aquifer system and intermediate

 confining unit in the middle of the local-scale model area.................................................................... 33

18. Simulated and measured hydraulic heads in the Upper Floridan aquifer in the middle of

 the local-scale model area............................................................................................................................ 34

19. Simulated potentiometric surfaces of the surficial aquifer system and Upper Floridan

 aquifer in the local-scale model area ......................................................................................................... 35

20. Graph showing calculated and measured sulfur hexafluoride concentrations in the 

 selected public-supply well and monitoring wells.......................................................................................... 37

21. Graph showing calculated and measured tritium concentrations in the selected 

 public-supply well and monitoring wells........................................................................................................... 39
vii

22-24. Maps showing:

22. Simulated area contributing recharge for the selected public-supply well and age

 of particles derived from simulated particle traveltimes........................................................................ 40

23. Simulated area contributing recharge for the selected public-supply well with simulated

 traveltimes, point sources of contaminants in the local-scale study area, and a recent 

 digital-orthographic-quarter-quadrangle depicting current land use/land cover........................... 42

24. Simulated area contributing recharge for monitoring wells and age of particles

 derived from simulated particle traveltimes.............................................................................................. 43

25. Graph showing estimated nitrate input concentrations, reconstructed nitrate in recharge

 concentrations, estimated nitrate concentrations at the selected public-supply well (PSW) 

 from flow-weighted particle tracking using median, minimum, and maximum denitrification

 rates, and measured nitrate concentrations at the selected PSW and surficial aquifer 

 system wells sampled in 2003 and 2004............................................................................................................. 44

26. Graph showing nitrate concentrations in the selected public-supply well sampled in 

 2003 and 2004 compared to calculated concentrations from 1992 to 2006 ............................................... 45

Appendix Figures

A1-A4. Maps showing:

A1. Updated estimates of horizontal hydraulic conductivity of the Upper Floridan aquifer

 for the regional model that was used to estimate flow at the boundary of the

 local-scale model............................................................................................................................................. 50

A2. Updated estimates of vertical hydraulic conductivity of the Upper Floridan aquifer for the 

 regional model that was used to estimate flow at the boundary of the local-scale model........... 51

A3. Updated estimates of recharge for the regional model that was used to estimate flow at 

 the boundary of the local-scale model....................................................................................................... 52

A4. Updated estimates of horizontal hydraulic conductivity for the surficial aquifer 

 (layer 1) in the regional model that was used to estimate flow at the boundary of the 

 local-scale model............................................................................................................................................. 53

Tables

1. Inventory of groundwater wells and surface-water sites used in the local-scale study area.................... 13

2. Summary of hourly water-level measurements obtained from monitoring wells, stormwater 

 retention ponds, and the Hillsborough River in the local-study area, December 11, 2003 to 

 November 17, 2005 ......................................................................................................................................................... 15

3. Summary of specified flux by model boundary and layer..................................................................................... 20

4. Average groundwater withdrawal rates from public-supply wells, industrial wells, and 

 agricultural wells in the local-scale study area during 2000 ............................................................................... 24

5. Hydraulic, recharge, and transport parameters by hydrogeologic unit and model layer used 

 in local-scale groundwater flow and particle-tracking simulations.............................................................25-26

6. Measured and simulated hydraulic heads and hydraulic head gradients, in the local-scale 

 model, and residual, weighted residual, aquifer, and data source for monitoring wells.........................31-32

7. Simulated water budget with inflow and outflow components to the local-scale modeled area .............. 36

8. Measured and simulated sulfur hexafluoride and tritium concentrations in the local-scale 

 model and residual, weighted residual, and aquifer name for monitoring wells............................................ 38

9. Simulated particle ages, minimum, median, and maximum at monitoring wells and at the 

 selected public-suppy well compared to the apparent age of the young fraction of water, 

 interpreted from sulfur hexafluoride and tritium concentrations by aquifer................................................... 39

10. Land-use statistics in the contributing recharge area for the selected public-supply well, 2000............... 41
viii

Conversion Factors

Multiply By To obtain

Length

centimeter (cm) 0.3937 inch (in.)

millimeter (mm) 0.03937 inch (in.)

meter (m) 3.281 foot (ft) 

kilometer (km) 0.6214 mile (mi)

Area

square kilometer (km2

) 247.1 acre

Volume

liter (L) 33.82 ounce, fluid (fl. oz)

liter (L) 0.2642 gallon (gal)

Flow rate

cubic meter per second (m3

/s) 70.07 acre-foot per day (acre-ft/d) 

cubic meter per second (m3

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

cubic meter per day (m3/d) 35.31 cubic foot per day (ft3/d) 

cubic meter per day (m3/d) 264.2 gallon per day (gal/d) 

cubic meter per second (m3

/s) 22.83 million gallons per day (Mgal/d) 

Hydraulic conductivity

meter per day (m/d) 3.281 foot per day (ft/d) 

Temperature

degree Celsius (°C) °F = (1.8 × °C) + 32 degree Fahrenheit (°F)

Vertical coordinate information is referenced to the North American Vertical Datum of 1988 (NAVD 88).

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

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

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

Acronyms and Abbreviations

ACR area contributing recharge

cm/yr centimeters per year

L/min liters per minute

mg/L milligrams per liter

NAWQA National Water-Quality Assessment Program

PCE tetrachloroethene

pptv volume fraction, in parts per trillion

PSW selected public-supply well

TANC Transport of Anthropogenic and Natural Contaminants

TTP-4 selected public-supply well in Temple Terrace, Florida

TU tritium units

VOC volatile organic compounds
Abstract

Shallow ground water in the north-central Tampa Bay 

region, Florida, is affected by elevated nitrate concentrations, 

the presence of volatile organic compounds, and pesticides 

as a result of groundwater development and intensive urban 

land use. The region relies primarily on groundwater for 

drinking-water supplies. Sustainability of groundwater 

quality for public supply requires monitoring and understanding of the mechanisms controlling the vulnerability of 

public-supply wells to contamination. A single public-supply 

well was selected for intensive study based on the need to 

evaluate the dominant processes affecting the vulnerability 

of public-supply wells in the Upper Floridan aquifer in the 

City of Temple Terrace near Tampa, Florida, and the presence of a variety of chemical constituents in water from the 

well. A network of 29 monitoring wells was installed, and 

water and sediment samples were collected within the area 

contributing recharge to the selected public-supply well to 

support a detailed analysis of physical and chemical conditions and processes affecting the water chemistry in the well. 

A three-dimensional, steady-state groundwater flow model 

was developed to evaluate the age of groundwater reaching the 

well and to test hypotheses on the vulnerability of the well to 

nonpoint source input of nitrate.

Particle tracking data were used to calculate environmental tracer concentrations of tritium and sulfur hexafluoride 

and to calibrate traveltimes and compute flow paths and 

advective travel times in the model area. The traveltime of 

particles reaching the selected public-supply well ranged from 

less than 1 day to 127.0 years, with a median of 13.1 years; 

nearly 45 percent of the simulated particle ages were less 

than about 10 years. Nitrate concentrations, derived primarily 

from residential/commercial fertilizer use and atmospheric 

deposition, were highest (2.4 and 6.11 milligrams per liter 

as nitrogen, median and maximum, respectively) in shallow 

groundwater from the surficial aquifer system and lowest 

(less than the detection level of 0.06 milligram per liter) in the 

deeper Upper Floridan aquifer. Denitrification occurred near 

the interface of the surficial aquifer system and the underlying 

intermediate confining unit, within the intermediate confining 

unit, and within the Upper Floridan aquifer because of 

reducing conditions in this part of the flow system. However, 

simulations indicate that the rapid movement of water from 

the surficial aquifer system to the selected public-supply 

well through karst features (sinkholes) and conduit layers 

that bypass the denitrifying zones (short-circuits), coupled 

with high pumping rates, allow nitrate to reach the selected 

public-supply well in concentrations that resemble those of 

the overlying surficial aquifer system. Water from the surficial 

aquifer system with elevated concentrations of nitrate and low 

concentrations of some volatile organic compounds and pesticides is expected to continue moving into the selected publicsupply well, because calculated flux-weighted concentrations 

indicate the proportion of young affected water contributing 

to the well is likely to remain relatively stable over time. The 

calculated nitrate concentration in the selected public-supply 

well indicates a lag of 1 to 10 years between peak concentrations of nonpoint source contaminants in recharge and 

appearance in the well.

Simulations of Groundwater Flow and Particle 

Tracking Analysis in the Area Contributing Recharge 

to a Public-Supply Well near Tampa, Florida, 2002-05

By Christy A. Crandall, Leon J. Kauffman, Brian G. Katz, Patricia A. Metz, W. Scott McBride, 

and Marian P. Berndt
2 Simulations of Groundwater Flow and Particle Tracking Analysis ... to a Public-Supply Well near Tampa, FL, 2002-05

Introduction

Groundwater provided public drinking-water supplies 

for 37 percent of the population of the United States in 

2000 (Hutson and others, 2004). Within the State of Florida, 

the reliance on groundwater for drinking water is much 

greater—80 percent of the population (nearly 13 million 

people) relied on groundwater for publicly supplied drinking 

water in 2000. The percentage of Florida’s population using 

publicly supplied drinking water from groundwater increased 

from 50 percent in 1950 to 80 percent in 2000 (Marella and 

Berndt, 2005). In 2000, 54 percent of the groundwater for 

public drinking water in Florida was supplied by the Floridan 

aquifer system. The Floridan aquifer system supplied public 

drinking water to 8.2 million people in four States (Alabama, 

Georgia, South Carolina, and Florida) in 2000 (Marella and 

Berndt, 2005). Nationwide, the Floridan aquifer system 

accounted for 8 percent of the groundwater withdrawn for 

publicly supplied drinking water (Maupin and Barber, 2005). 

One of the most densely populated areas in Florida relying 

on groundwater is the Tampa Bay area (fig. 1). In 2000, this 

area withdrew more than 1,135 million liters per day, with 74 

percent of that from the Floridan aquifer system.

In 2001, the U.S. Geological Survey (USGS) National 

Water-Quality Assessment (NAWQA) Program, began an 

intensive study to assess the vulnerability of public-supply 

wells to contamination from a variety of compounds (Eberts 

and others, 2005). The Transport of Anthropogenic and Natural 

Contaminants (TANC) study is focusing on the transport and 

chemical processes of selected naturally occurring and anthropogenic contaminants from urban and agricultural sources within 

that part of the groundwater system contributing water to publicsupply wells. Data collection for this study was completed in 

November 2005. Because subsurface processes and management practices differ among aquifers and public-water systems, 

public-supply wells in different parts of the Nation are not 

equally vulnerable to contamination, even where similar 

contaminant sources exist. The TANC study is identifying and 

comparing these important differences in a complementary set 

of aquifer systems, urban settings, and public-water systems 

based on data that were collected and analyzed using consistent 

methods. The northern part of the Tampa Bay region is one of 

the selected sites for a local-scale comprehensive study.

Purpose and Scope

The purpose of this report is to discuss the dominant 

processes affecting the vulnerability to contamination of a 

public-supply well in the Upper Floridan aquifer in the City 

of Temple Terrace near Tampa in north-central Florida. This 

report includes a brief description of the design of the local 

monitoring well network and methods of data collection and 

analysis. Groundwater ages and the distribution of selected 

constituents are used for developing basic interpretations of 

groundwater recharge and discharge patterns and pathways. 

This report documents the construction and calibration of a 

local-scale groundwater flow model and the approaches used 

to refine a previously developed regional-scale model. The 

results of simulations of groundwater flow and traveltimes are 

also described. This report is intended to serve as a foundation for model synthesis analyses on which results can be 

compared between this TANC local-scale study area and other 

TANC study units in Connecticut, California, and Nebraska. 

Results of synthesis analyses and descriptions of other TANC 

study units are not included in this report.

Previous Studies

The hydrogeology of the Floridan aquifer system has 

been studied throughout its entire extent and locally in westcentral Florida and the Tampa Bay region. Miller (1986) and 

Bush and Johnston (1988) characterized the hydrogeologic 

framework and hydrology of the Upper Floridan aquifer 

system throughout its regional extent, and Ryder (1985) 

characterized the hydrology of the Upper Floridan aquifer 

in west-central Florida. Stewart and others (1978) examined 

factors affecting the availability and quality of groundwater in 

the Temple Terrace area.

Current and historical surface-water data for southwestern Florida have been summarized in Coffin and Fletcher 

(2001). Flood profiles and historical information are discussed 

in Turner (1974) and by Goetz and others (1978) for the 

Hillsborough River Reservoir and by Murphy (1978) for 

Cypress Creek (a tributary to the Hillsborough River). Land 

use and historical discharge and nutrient loading to the lower 

Hillsborough River were discussed in Stoker and others 

(1996). Wolansky and Thompson (1987) described groundwater and surface-water interaction between the Hillsborough 

River and the Upper Floridan aquifer, and the Hillsborough 

River wetlands were characterized by Lewelling (2004).

Within the Tampa Bay region, groundwater flow models 

have been developed to study the groundwater flow system. 

A regional transient groundwater flow model was developed 

for the central-north Tampa Bay region by SDI Environmental 

Services, Inc. (1997). Langevin (1998) used this groundwater flow and particle tracking model, but added a fracture 

generator that increases transmissivities in fracture locations to 

approximate groundwater traveltimes in karst terrains. Yobbi 

(2000) optimized parameters for a steady-state version of this 

central-north Tampa Bay regional groundwater flow model, 

which was used as the initial model for the TANC regionalscale study. The regional-scale model was documented in 

a recent report on the hydrogeology of the Upper Floridan 

aquifer in the Tampa Bay region, which included regional 

information on climate, geology, hydrogeology, and water 

supply and the simulated contributing areas along with results 

of chemical analyses of groundwater (Crandall, 2007). The 

purpose of that regional-scale investigation was to identify 

major factors influencing contaminant occurrence and transport and to simulate areas contributing recharge (ACR) on a 

regional scale using an existing groundwater flow model.
Introduction 3

0 1 2 3 4 KILOMETERS

0 1 2 MILES

TTP-4

(PSW)

Hillsborough

River

Reservoir

Cypress Creek

Tampa Bypass Canal

House

Hillsborough River

City of Temple Terrace

Local-scale model boundary

Major streams

EXPLANATION

Area shown in figure 7

Area shown in figures 17 and 18

Area shown in figures 22-24

Cow

Creek

EXPLANATION

Aquifer Confinement Status

Unconfined

Semiconfined

Confined

Regional Model Boundary

Local-scale Model

Boundary

Northern Extent of the

Floridan Aquifer

Major Cities

Tampa

Ocala

Miami

Orlando

Pensacola

Tallahassee

Gainesville

Jacksonville

0 100 200 500 KILOMETERS

Tampa Bay

28°

29°

30°

27°

26°

25°

24°

31°

32°

33°

89° 88° 87° 86° 85° 84° 83° 82° 81° 80° 34° 79°

300 400

0 100 200 300 MILES

Figure 1. Regional extent of the Floridan aquifer 

system, aquifer confinement status, major 

cities in Florida, and the TANC regional- and 

local-scale model boundaries in the Tampa Bay 

area. TANC is Transport of Anthropogenic and 

Natural Contaminants.
4 Simulations of Groundwater Flow and Particle Tracking Analysis ... to a Public-Supply Well near Tampa, FL, 2002-05

Knochenmus and Robinson (1996) compiled results from 

numerous aquifer tests and other hydraulic tests in the Tampa 

Bay region. Initial measured values of hydraulic conductivity, 

which were used as the basis for simulated parameter values, 

were obtained from Knochenmus and Robinson (1996), Yobbi 

(2000), and the Florida Geological Survey (Jon Arthur, written 

commun., 2003). Knochenmus and Robinson (1996) also 

used various schemes to assess the effects of karst features on 

contributing area size, shape, and transport times. Sinkholes 

and their effect on water chemistry of the Upper Floridan 

aquifer were studied by Trommer (1987).

A study in New Jersey developed the general methodology 

applied by the TANC study to assess contaminant transport, 

ages, and flow paths (Kauffman and others, 1998; Stackelberg 

and others, 2000). A comparison of methods that assessed 

areas contributing recharge to wells was compiled by Franke 

and others (1998).

Description of Study Area

In 2002, a community water system public-supply well 

(TTP-4) in Temple Terrace, Florida, was selected as 1 of 

30 wells which were sampled for the NAWQA Program’s 

Source Water Quality Assessment study in the Tampa area. 

Water samples from this selected public-supply well (hereafter referred to as the PSW) were initially collected and 

analyzed in the fall of 2002 for a wide suite of chemical and 

physical constituents. Results from the analyses showed the 

presence of multiple contaminants, but concentrations were 

below drinking-water standards. The analyses detected six 

volatile organic compounds (VOCs) and four pesticides, as 

well as elevated nitrate concentrations, arsenic, radon-222, 

and uranium (Katz and others, 2007). The construction 

and operational practices of this PSW are representative 

of many public-supply wells that use the Upper Floridan 

aquifer for supplying the population of northern Tampa Bay, 

which includes Hillsborough, Pinellas, Pasco, and Hernando 

Counties. In addition, Temple Terrace was willing to make 

available one of their primary community water-system 

public-supply wells. The combination of the detected constituents, cooperation of the city utility, and typical well construction and operational practices prompted the selection of this 

PSW for the local-scale study.

The local-scale study area, which is the focus of this 

report, is located in west-central peninsular Florida within 

Temple Terrace, in the northern Tampa Bay region (figs. 

1 and 2). The local-scale study area is 86.25 km2 and is 

underlain by the Floridan aquifer system. This aquifer 

system underlies much of the southeastern United States, and 

its main productive zone, the Upper Floridan aquifer, is a 

major drinking-water source for nearly 10 million people in 

Alabama, Florida, Georgia, and South Carolina (Marella and 

Berndt, 2005), including the Tampa Bay region and Temple 

Terrace. The local-scale study area is bounded on the west 

and southwest by Tampa. The local-scale study area and the 

local-scale model area are identical in boundary and area. 

The local-scale study area and model boundary were determined by using the regional-scale model to identify the ACR 

of the PSW and bounding the likely ACR with the local-scale 

model area. The model area is oriented to incorporate the 

primary karst feature orientation, which would minimize error 

in cell-to-cell flows.

The Upper Floridan aquifer varies from unconfined 

to confined conditions in the local-scale study area but is 

mainly classified as unconfined or semiconfined. Numerous 

karst features are present, including sinkholes and solutionenlarged fractures and bedding plains. About 35 sinkholes 

were reported in the local-scale study area; however, many 

more sinkholes probably are present and are buried, filled, 

or not reported. Several springs are present within or near 

the local-scale study area (fig. 2). The local-scale study area 

is oriented around two sharp bends of the Hillsborough River 

and includes a terraced area that trends from the northwestern 

study area boundary to the southeast toward the river in the 

center of the local-scale study area (fig. 3). The local-scale 

study area also includes floodplains near the river, sinkholes 

throughout, and deeper sinkholes on the terrace. Most of 

the land use in the local-scale study area is residential with 

commercial strips along major roadways (fig. 2). Tributaries 

include Cypress Creek, which is located in the northern 

wetlands area, Cow House Creek (figs. 2 and 3). The Tampa 

Bypass Canal removes water from the Upper Floridan 

aquifer.

Climate

The climate in the northern Tampa Bay region is 

characterized by warm, wet summers and relatively dry, 

mild winters, with an average annual temperature for Tampa, 

Florida, of 22.4 o

C (Owenby and Ezell, 1992). Average 

rainfall for Temple Terrace between 1993 and 2005 (Mike 

Darrow, City of Temple Terrace, written commun., 2005) was 

152.50 cm/yr, but was as low as 101 cm/yr during extreme 

drought conditions in 2000 and as high as 205 cm/yr during 

extreme wet conditions in 2004. Rainfall amounts vary 

seasonally with more than half of the total annual rainfall 

usually occurring between June and September. The wettest 

months are usually August and September (average of 23.33 

and 23.67 cm, respectively, from 1993 to 2005) with most 

of the rainfall occurring as afternoon thunderstorms, tropical 

storms, and hurricanes. On average, this region experiences 

high evapotranspiration rates of 114 to 150 cm/yr, as occurred 

during 1962-92 (Owenby and Ezell, 1992), and high recharge 

of up to 51 cm/yr. Pan evaporation rates average 125 to 

150 cm/yr (Farnsworth and others, 1982).
Description of Study Area 5

0 1 2 3 4 KILOMETERS

0 1 2 MILES

TTP-4 (PSW)

Hillsborough River Reservoir

Tampa

Cypress Creek

New Tampa

Temple

Terrace

Golf Course

Sulphur Springs

02304500

Tampa Bypass Canal

Cow House Creek

Hillsborough River

Lettuce Lake, and

Eureka Springs

02304000

Original Supply Spring

Temple Terrace city limits

Local-scale model

boundary

Major streams

EXPLANATION

TTP-4 (PSW)

Gaging station and number

Spring

Dam

583

582

Figure 2. Local-scale study area projected over the digital-orthographic-quarter quadrangle showing City 

of Temple Terrace boundary, nearby cities, springs, streams, gaging stations, and the selected public-supply 

well (PSW).
6 Simulations of Groundwater Flow and Particle Tracking Analysis ... to a Public-Supply Well near Tampa, FL, 2002-05

Population and Land Use

Temple Terrace, nested within suburban Tampa, has 

grown from a small, isolated town of 29 residents in 1920, to 

433 residents in 1950, to a city of more than 23,000 residents 

in 2000 (City of Temple Terrace, 2006). The city was one of 

the Nation’s first planned communities, with parks and a golf 

course incorporated into the original design of the town. Some 

citrus groves were still located within the city limits until the 

1950s (Stewart and others, 1978); however, the predominant 

land use changed from primarily citrus agriculture to residential/commercial between 1920 and 1975. Land use has 

remained relatively stable since that time at about 84 percent 

urban (mostly residential), 6 percent rangeland, 4 percent 

wetland, and 4 percent agriculture (Homer and others, 2000). 

Commercial development has occurred mainly along major 

corridors. Other major land-use/land-cover features include 

an 18-hole golf course, which runs parallel to the northeastern 

bank of the Hillsborough River, and numerous residential 

developments built around sinkhole lakes. Paved areas, strip 

malls, and industrial land uses are also present in the study 

area.

Physiography

The local-scale study area is located in the coastal 

lowlands region of the southeastern United States. The coastal 

lowlands physiographic province consists of a relatively 

flat plain bounded by an erosional escarpment formed by 

the remnant Wicomico Terrace (fig. 3; White, 1970). The 

terrace generally parallels the coast, having been formed by 

seas that once stood at higher levels than at present. This 

dominant physiographic feature includes a sandy ridge on 

which lies much of the City of Temple Terrace. Land-surface 

elevation ranges from 24.4 m on the western side of the 

Projected Sinkholes

Remnant Wicomico

Terrace

Northern

Wetlands

28°00’00”

82°22’30”

28°00’00”

82°22’30”

EXPLANATION

Elevation, in meters

above NAVD 88

High : 27.78

Low : 0.18

0

2

3

1

4 KILOMETERS

0

1

2 MILES

Figure 3. Digital Elevation Model (DEM) of the 

local-scale study area with 20x exaggeration of vertical profile 

of the local-scale study area, showing the remnant Wicomico Terrace, 

numerous sinkholes projected from the DEM and the Northern Wetlands.
Description of Study Area 7

terrace, to 27.4 m along the central part of the terrace (Stewart 

and others, 1978), and decreases to less than 6 m near the 

river; average elevation is about 15 m. Other dominant 

physiographic features in the local-scale study area include 

the Hillsborough River floodplain, which is deeply incised 

(3-6 m) in the valley (Stewart and others, 1978). Sinkholes 

as deep as 6 to 8 m are also common in the local-scale study 

area (fig. 3; Trommer, 1987), and many have been filled and 

used for stormwater retention ponds. Sandy soils that cover 

the local-scale study area are well drained and have relatively 

deep water tables, rapid percolation, internal drainage, and 

high recharge potential (HydroGeoLogic, Inc., 1997).

Surface-Water and Groundwater Interactions

The Hillsborough River meanders through the localscale study area, from the northeastern to the southwestern 

boundaries, and discharges to Hillsborough Bay (a part of 

the greater Tampa Bay) about 16 km downstream from the 

local-scale study area (figs. 1 and 2). Much of the channel is 

defined and controlled by karst features (Lewelling, 2004). 

Average annual flow was recorded to be 12.8 m3

/s between 

1939 and September 2000, the period of record (Coffin and 

Fletcher, 2001). Natural flow in the Hillsborough River was 

disrupted in the 1920s when a hydroelectric dam was installed 

near the southwestern edge of the local-scale study area. The 

Hillsborough River and its tributaries, Cow House Creek 

and Cypress Creek (fig. 2) in the Temple Terrace area, are 

affected by backwater from the Hillsborough River Reservoir 

(Fernandez and others, 1984) so that flow to and from the river 

is largely controlled by reservoir stage.

The Hillsborough River is in direct hydraulic contact 

with the surficial aquifer system and the Upper Floridan 

aquifer in the local-scale study area. The river usually gains 

water from groundwater discharge as it meanders through the 

northern part of the local-scale study area; however, some 

gains in surface water through upward leakage may be lost to 

evapotranspiration, especially in the northern wetlands (Goetz 

and others, 1978). Below the Fowler Avenue gaging station 

(station number 02304000 in fig. 2), the Hillsborough River 

occasionally loses water to the Upper Floridan aquifer during 

high-flow events (Wolansky and Thompson, 1987).

Hydrogeologic Setting

The local-scale study area is underlain by three primary 

hydrogeologic units that include the surficial aquifer system, 

intermediate confining unit, and Floridan aquifer system; the 

local- and regional-scale studies primarily include the Upper 

Floridan aquifer (fig. 4). The Lower Floridan aquifer is present 

in the regional-scale study area, but is separated from the Upper 

Floridan aquifer by a middle confining unit; the Lower Floridan 

aquifer is not present in the local-scale study area (Miller, 1986). 

Each hydrogeologic unit is defined by specific geologic units 

that give each unique hydraulic and hydrogeologic characteristics and guide groundwater flow model layer design.

Geology

The local-scale study area is underlain by sand, clayey 

sand, sandy clay, clay, and carbonate rocks that were deposited in primarily marine environments during the middle 

and late Eocene to Holocene ages of the Tertiary Period of 

the Cenozoic Era. During this time, changes in sea level 

produced terraces and marine environments in areas that are 

dry land today. The geologic framework is characterized by 

multiple layers of sand to clayey sand to sandy clay to clay 

that overlie a highly weathered thick limestone sequence 

containing numerous dissolution (karst) features. Numerous 

localized surface or buried depressions (collapse features or 

sinkholes) disrupt the layered horizontal geologic framework 

and form vertical conduit features in layers where dissolution 

has occurred. In addition, dissolution features occur along 

the bedding plain and bedding-plain fractures that also add to 

the karst character of the Upper Floridan aquifer. Fractures 

are bimodal along two major axes of orientation with the 

most common orientation at about 310° and the second most 

common at 40°. The fracture orientations are approximately 

parallel and perpendicular to the major structural element 

(Ocala Platform) of the Florida Peninsula in the regional- and 

local-scale study areas (Schmidt, 1994). Numerous springs are 

present in the streambed of the Hillsborough River directly 

outside the local-scale study area (fig. 2).

Lithology 

The lithology of the surficial sediments, clay unit, and 

limestone that compose the hydrogeologic units (surficial 

aquifer system, intermediate confining unit, and Upper 

Floridan aquifer, respectively) were described based on 

analysis of well cuttings, split-spoon coring, and grain-size 

analysis (Katz and others, 2007). Mineralogical analyses of 

selected cores were also performed using x-ray diffraction and 

mass spectrometery. Details about the lithology, depth of saturation, and results of the grain-size analysis for selected wells 

are presented in Katz and others (2007) and are summarized in 

the following paragraphs.

The uppermost surficial sediments are a well sorted, very 

fine to fine quartz sand of Holocene age, and generally less 

than 3 m thick (95 percent). The remaining surficial sediment 

consists of silts and clays with thicknesses that range from 

about 6 to 15 m thick. Surficial sediments are thinnest on 

the eastern boundary of the local-scale study area near the 

Hillsborough River, and thickest in the middle and along the 

western edge of the local-scale study area where sand terraces 

are present. The sand is white to buff colored near the surface 

and contains a mixture of organic matter and silt. Below the 

organic layer, the sand is stained a pale-yellow-orange color, 
8 Simulations of Groundwater Flow and Particle Tracking Analysis ... to a Public-Supply Well near Tampa, FL, 2002-05

which is probably due to the influence of iron leaching into 

the shallow groundwater system. The clay content gradually increases, creating a sequence of clayey sand generally 

less than 1 m thick. The sand content ranges from 52 to 95 

percent, and the remaining sediment is composed of silt and 

clay. The clayey sand sequence is the most variable in terms 

of mineralogy, which probably is due to the variability in the 

clay content.

Below the clayey sand sequence, the clay content 

increases, grading to a sequence of sandy clay of the 

Hawthorn Group of late Miocene age, which corresponds to 

the intermediate confining unit. The intermediate confining 

unit in the local-scale study area varies in clay content, 

consistency, color, composition, and permeability. Generally, 

the unit has a dense plastic consistency, is tan or greenish-gray 

or orange-red, and contains varying amounts of sand, chert, 

and carbonate mud. The clay may be calcareous in places, 

particularly near the underlying limestone contact. Thickness 

ranges from 0 to less than 2 m in the local-scale study area. 

Carr and Alverson (1959) and Sinclair (1974) describe the clay 

of the intermediate confining unit as a weathered residuum of 

the underlying Tampa Member limestone.

The highly weathered limestone of the Tampa Member of 

the Arcadia Formation of the Hawthorn Group underlies the 

intermediate confining unit in the local-scale study area and is 

identified as the top of the Upper Floridan aquifer. The Tampa 

Member of early Miocene age varies from tan to white, is soft 

to hard, usually sandy, and fossiliferous; it commonly contains 

clay lenses and cavities. The Tampa Member is generally less 

than 30 m thick, based on well cuttings collected from 14 

wells throughout the local-scale study area, and the depth of 

the top of the Tampa Member ranges from 3.5 to 12 m below 

land surface. For two wells, the sands and clays were found 

far below the typical depth to limestone, which suggests that 

Suwannee Limestone

Ocala Limestone

Avon Park Formation

Undifferentiated Sands

and Clays

Quartz sand, silty sand, clayey sand, peat, shell

Clay, minor quartz sand, phosphate, fine-grained dolomite,

residual limestone

Limestone, minor quartz sand, phosphate, chert, clay,

fine-grained dolomite

Limestone, packstone to grainstone, trace quartz sand,

organics, variable dolomite and clay content, highly

fossiliferous, vuggy

Limestone, micritic, chalky, very fine to fine-grained, soft,

poorly indurated, trace organics, clays and dolomite,

abundant foraminifera

Evaporites occur in dolomite as interstitial gypsum

and anhydrite with evaporite filling pore space and as

interbeds in the lower part

Dolomite is hard, brown, sucrosic in texture and

commonly fractured

Limestone, dolomite, and evaporites; Limestone and

dolomite interbeds typical in upper part, deeper beds are

continuous dolomite with increasing evaporites at base

Limestone is fine-grained, tan, recrystallized packstone

with variable amounts of organic-rich laminations near top

Intermediate

confining unit

Tampa/Suwannee

producing zone

Ocala semiconfining

unit

Ocala/Avon Park

producing zone

Avon Park

producing zone

Upper Floridan aquifer

Middle confining unit

Surficial aquifer system

Hawthorn Group

Tampa Member

of the

Arcadia Formation

Holocene

to

Pliocene

Miocene

Oligocene

Eocene

SERIES STRATIGRAPHIC

UNIT GENERAL LITHOLOGY HYDROGEOLOGICUNIT MODEL LAYER

Layers 5-7

Layers 9-11

Layers 1-3

Layer 4

Layer 8

Layer 12

Layer 13

Below the freshwater

production zone

Figure 4. Geologic and hydrogeologic stratigraphic column.
Description of Study Area 9

infilling of overburden materials into cavities has occurred. 

This infilling of limestone cavities increases the interconnection of overlying units to the Upper Floridan aquifer. Within 

the Tampa Member is a horizontally extensive layer containing 

expansive conduits, fractures, and (or) bedding plain erosional 

features. This information is based on well logs and the nearly 

instantaneous drawdown experienced by the selected PSW 

when other public-supply wells in the area open to this zone 

are turned on. Also, many conduits were noted in three wells 

open to this layer: wells PSW, MAS-R‐F160, and WP‐F150.

The Suwannee Limestone of Oligocene age in the Upper 

Floridan aquifer underlies the Tampa Member, and its thickness is about 60 to 80 m in the local-scale study area. The 

limestone is pure calcium carbonate in composition, white 

to light tan, soft and granular, and contains abundant fossil 

detritus and organic structures such as casts, molds, and 

borings of mollusks and tests of foraminifera and bryozoans 

(Carr and Alverson, 1959). Many fossil molds within this rock 

unit result in a high porosity.

Below the Suwannee Limestone are the Ocala Limestone 

of late Eocene age and the Avon Park Formation of early to 

middle Eocene age (Miller, 1986). Both are part of the Upper 

Floridan aquifer. The Ocala Limestone is a soft, friable, white 

coquina composed of fossil foraminifera, echinoid, and bryozoans fragments loosely bound in a micritic cement. This unit 

is also highly porous and dissolution features are abundant. 

The Avon Park Formation is another highly fractured, karstic, 

porous, dolomitic limestone with very high permeability due 

to the karst features. The freshwater interface, defined as 

the level of 5,000 mg/L chloride concentration in the Upper 

Floridan aquifer, occurs in the Avon Park Formation in the 

local-scale study area. The combined thickness of the Ocala 

Limestone and Avon Park Formation above the 5,000-mg/L 

chloride line ranges from 72.5 to 105.0 m with a median of 

88.7 m. The entire Upper Floridan aquifer freshwater flow 

system lies above the middle confining unit. The freshwater/

saltwater interface is a no-flow boundary (Reilly, 2001). 

Only the freshwater part of the flow system is discussed in 

this report.

Groundwater Flow System

The potentiometric surface of the surficial aquifer system 

generally follows land-surface elevations and is mostly saturated 

in areas where land elevations are low and not present on the 

terraces. The depth to the water table is variable and ranges from 

about 3 to 16 m below land surface. The surficial aquifer system 

is recharged primarily by the infiltration of rainfall. Recharge is 

relatively rapid because of the highly permeable surficial sands. 

The surficial aquifer system is not used as a source of water 

supply in the study area because of the relatively low yields to 

wells of less than 19 L/min.

Because of its low permeability, the intermediate 

confining unit retards the downward leakage between the 

surficial aquifer system and Upper Floridan aquifer. The 

occurrence, thickness, and permeability of this unit are highly 

variable throughout the local-scale study area. Breaches form 

in this unit due to the dissolution and collapse of the underlying limestone of the Upper Floridan aquifer, facilitating 

preferential pathways for leakage to the Upper Floridan 

aquifer.

In the Upper Floridan aquifer, most of the production 

wells in the local-scale study area are open to the Tampa 

Member or the Suwannee Limestone. Wells that are deeper 

than about 100 m probably tap the upper part of the Ocala 

Limestone and (or) Avon Park Formation (Stewart and others, 

1978). The total thickness of the active flow system of the 

Upper Floridan aquifer ranges from 200 to 280 m in the 

local-scale study area.

Production and monitoring wells in Hillsborough 

County have open intervals of relatively large diameters, 

usually greater than 3 m (Stewart and others, 1978). These 

large diameters intersect solution openings and cavities in 

most wells. The selected PSW is open to one of these highly 

productive, cavernous, water-yielding zones in the Tampa 

Member of the Upper Floridan aquifer. The top of the highly 

productive cavernous zone is present at about 36 to 55 m 

below land surface (Stewart and others, 1978). Wells tapping 

this zone can yield as much as 5,700 L/min.

Groundwater in the study area flows downward and 

laterally through the surficial aquifer system until it reaches 

the intermediate confining unit. Water then moves laterally 

along the contact between the surficial aquifer system and 

intermediate confining unit until it encounters a surface-water 

feature (river, stream, or depression) or breaches into the 

intermediate confining unit. Breaches serve as preferential 

flow paths for leaking water to the Upper Floridan aquifer 

and allow water to short circuit slower flow paths through the 

intermediate confining unit. Water that leaks into the Upper 

Floridan aquifer may move slowly in the porous matrix or 

through secondary porosity features, such as conduits. The 

water then travels rapidly to wells, springs, rivers, streams or 

to a large regional discharge feature, such as Tampa Bay or the 

Gulf of Mexico. The conceptual groundwater flow system is 

presented in figure 5.

The groundwater flow direction in the surficial aquifer 

system is approximately from the northwest to southeast in the 

local-scale study area (fig. 6) and reflects land-surface elevation and local discharge features. The potentiometric surface 

of the surficial aquifer system is generally above the Upper 

Floridan aquifer, and highest in a well in the west-central 

part of the local-scale study area on the remnant Wicomico 

Terrace. The water level in this well also may be affected by 

water levels in a nearby stormwater retention pond. Generally, 

water levels in storm retention ponds are far above those of the 

surficial aquifer system and the Upper Floridan aquifer.
10 Simulations of Groundwater Flow and Particle Tracking Analysis ... to a Public-Supply Well near Tampa, FL, 2002-05

Groundwater flow direction in the Upper Floridan aquifer 

is also approximately from north to south or southeast in 

the local-scale study area (fig. 6). Locally, pumping affects 

hydraulic heads in the Upper Floridan aquifer, most noticeably in the zone that most of Temple Terrace public-supply 

wells are opened to (the highly transmissive cavernous zone). 

Pumping of production wells causes local instantaneous 

drawdown adjacent to production wells. The Upper Floridan 

aquifer appears to be in direct hydraulic connection with the 

Hillsborough River. Near the river, the groundwater flow 

direction depends upon reservoir operations. For example, 

before a major rainfall event (such as a hurricane), the 

reservoir water levels are lowered so the groundwater flow 

direction is toward the river. During drought conditions, the 

reservoir levels are kept high so that Tampa can continue to 

draw water for supply; the groundwater flow direction may be 

away from the river into the aquifer.

Methods

The methods used for this study were consistent with the 

objectives of the NAWQA/TANC study, which included the 

use of similar methods in all four local-scale study area investigations. The uses of MODFLOW to simulate groundwater 

flow, MODPATH to simulate particle tracking, and similar 

methods of water-quality data collection and analyses provide 

consistency and comparability for the TANC local-scale study.

Design of Sampling Network and Data 

Collection and Analysis

A total of 15 wells were installed in 2003-04 in five 

nests with three wells in each nest oriented along one of the 

general directions of groundwater flow to the selected PSW 

based on the regional-scale model. These nests were: Railway 

Park (RP), Recreation Center at 113th street (113RC), Gillette 

Recreation Center (GARC), Terrace Hills Center (THC), 

and Water Plant (WP) (fig. 7; table 1). Most of the nests 

consisted of one well screened in the surficial aquifer system, 

one well screened in the top of the Upper Floridan aquifer, 

and one well screened deeper in the Upper Floridan aquifer. 

At well nest 113RC, one well was screened in the intermediate confining unit.

During the summer of 2004, an additional 14 monitoring 

wells were installed, which provided the study with a total of 

29 monitoring wells. However, one well was dry, so the final 

total was reduced to 28 (fig. 7; table 1). The locations of these 

additional monitoring wells were selected based on updated 

estimates of the mapped surface expression of the area 

contributing water to the selected PSW. Three additional well 

nests were installed, including MAS, Lightfoot Retention Park 

(LRP), and 62nd Street Retention Pond (62SRP). Two wells 

were installed at Lynwood Park (LP)—one screened in the 

surficial aquifer system and the other screened in the intermediate confining unit. Three shallow wells were screened in the 

surficial aquifer system and installed at Queensway Retention 

UPPER FLORIDAN AQUIFER

Surficial aquifer system

Intermediate confining unit

Sinkhole

SW NE

REGIONAL

GROUNDWATER

DISCHARGE

FEATURE

DIRECTION OF REGIONAL

GROUNDWATER FLOW

Well

Wetland

Precipitation

Figure 5. Conceptual diagram of recharge, hydrogeology, and groundwater flow system dynamics for the local-scale study area.
Methods 11

Maximum, 16.73

Minimum, 4.90

5.32

7.97

7.70 8.19

8.25

8.18

13.92

4.16

3.97

4.48

4.56

4.56 5.61

4.54

4.43 5.19

5.37

5.92

5.52

6.87

6.37

7.41

11.91

Projected water table

elevation, in meters

Local-Scale Model

Boundary

Contour line – Shows

elevation of the water

table

Well – number is median

measured water level

for 2000

13.92

EXPLANATION

Maximum, 17.50

Minimum, 3.97

Projected potentiometric surface

of Upper Floridan aquifer, in meters

Local-Scale Model

Boundary

Contour line – Shows

projected potentiometric

surface of the Upper

Floridan aquifer

Well – number is median

measured water level for

2000

EXPLANATION

4.43

0 1 2 4 KILOMETERS 3

0 1 2 MILES

A

B

0 1 2 4 KILOMETERS 3

0 1 2 MILES

Figure 6. Median 

measured water 

levels and simulated 

potentiometric 

surfaces of the 

(A) surficial aquifer 

system and 

(B) Upper Floridan 

aquifer in the localscale study area, 

2000.
12 Simulations of Groundwater Flow and Particle Tracking Analysis ... to a Public-Supply Well near Tampa, FL, 2002-05

Park (QRP), Bonnie Brae Park (BBP), and Jaqueline Arbor 

(JARP). Surface-water samples were collected from the 

Hillsborough River (fig. 7) and from stormwater retention 

ponds at LRP, 62SRP, and MAS (fig. 7). Single wells at the 

MAS and LRP sites are screened in the intermediate confining 

unit. Figure 8 presents two hydrogeologic sections through 

the local-scale study area based on the drillers’ logs and core 

analyses.

Water levels were measured in all monitoring wells 

during water-quality sampling, and water-level recorders were 

installed in 14 monitoring wells, 3 retention ponds, and the 

Hillsborough River at Railway Park to provide water-level 

data that were used to calibrate the groundwater flow model. 

Groundwater levels and river stage were recorded hourly from 

December 2003 until November 2005. The median water-level 

value, calculated from the hourly groundwater levels or water 

582

583

0 0.2 0.3 MILES

0 0.2 0.5 KILOMETERS

82°23'30" 82°23' 82°22'30" 82°22'

28°02'30"

28°03'

Hillsborough River

LRPLRPLRPLRP-POND

S25

H105

F160

S30

H40

LPLPWP-S64

F150

F299

WPWPMAS-R-S30

F64

F160

POND

MAS-RMAS-RMASTTP-4

(PSW)

BBP-S45

62SRPS34

H55

F160

POND

JARP-S40

THC-SA/H46

F75

F197

THCTHCGARC-S23

F75

F200

GARCGARC- RP-S20

F77

F103

RPRP113RC-S35

H50

F190

113RC113RCQRP-S20

EXPLANATION

HYDROGEOLOGIC CROSS SECTION B-B'

HYDROGEOLOGIC CROSS SECTION A-A'

DIRECTION OF GROUNDWATER FLOW IN

UPPER FLORIDAN AQUIFER

SAMPLING LOCATION FOR WELL OR POND

AND SITE IDENTIFIERS BBP-S45

A

A'

A A'

B

B'

B B'

0.1 0.3 0.4

0.1

0 1 2 KILOMETERS

0 1 MILE

TTP-4

(PSW)

Hillsborough

River

Area

shown

above

City of

Temple

Terrace

Local-scale

model

boundary

Greco

Sink

Figure 7. Locations of wells and test holes installed for 

this study and trace of hydrogeologic sections A-A’ and 

B-B’. PSW is the selected public-supply well.
Methods 13

Table 1. Inventory of groundwater wells and surface-water sites used in the local-scale study area.

[Well locations shown in fig. 7; NA, not applicable]

Site name Short site name Station identifier

Water 

level

recorder

Measuring 

point

 altitude, 

(meters)

Well 

depth, 

(meters)

Terrace Hill Circle, Upper Floridan aquifer (lower) THC-F197 280241082224401 No 15.99 60.05

Terrace Hill Circle, Upper Floridan aquifer (upper) THC-F75 280241082224402 No 15.91 22.86

Terrace Hill Circle, surficial aquifer system THC-SA/H46 280241082224403 No 15.83 13.72

Railway Park, Upper Floridan aquifer (lower) RP-F103 280249082220701 Yes 11.16 31.39

Railway Park, Upper Floridan aquifer (upper) RP-F77 280249082220702 Yes 11.12 22.86

Railway Park, surficial aquifer system RP-S20 280249082220703 Yes 11.15 6.1

Railway Park, Hillsborough River HRIVER 280244082220200 Yes 7.17 NA

Temple Terrace Recreation Center at 113th, Upper Floridan aquifer 113RC-F190 280301082222701 No 12.81 60.96

Temple Terrace Recreation Center at 113th, intermediate confining unit 113RC-H50 280301082222702 No 12.73 15.24

Temple Terrace Recreation Center at 113th, surficial aquifer system 113RC-S35 280301082222703 No 12.8 10.67

Temple Terrace Recreation Center at Gillette, Upper Floridan aquifer (lower) GARC-F200 280253082223801 Yes 12.42 60.96

Temple Terrace Recreation Center at Gillette, Upper Floridan aquifer (upper) GARC-F75 280253082223802 No 12.48 22.71

Temple Terrace Recreation Center at Gillette, surficial aquifer system GARC-S23 280253082223803 Yes 12.23 7.01

Temple Terrace Water Plant, Upper Floridan aquifer (lower) WP-F299 280247082231901 Yes 24.49 91.44

Temple Terrace Water Plant, Upper Floridan aquifer (upper) WP-F150 280247082231902 Yes 24.51 45.72

Temple Terrace Water Plant, surficial aquifer system WP-S64 280247082231903 Yes 24.38 19.81

Temple Terrace City Well No. 4 at Temple Terrace FL TTP-4 (PSW) unpublished No 24.65 53.04

MAS Residence, Upper Floridan aquifer (Lower) MAS- R-F160 280242082232401 Yes 20.33 48.77

MAS Residence, surficial aquifer system MAS-R-S30 280242082232403 No 20.31 9.14

MAS Residence, Upper Floridan aquifer (Upper) MAS-R-F64 280242082232403 Yes 20.34 19.51

MAS Retention Pond MAS-POND 280242082232900 Yes 15.99 NA

62nd St. Retention Pond, Upper Floridan aquifer (lower) 62SRP-F160 280241082230701 No 21.55 48.77

62nd St. Retention Pond, Upper Floridan aquifer (upper) 62SRP-H55 280241082230702 No 21.67 16.76

62nd St. Retention Pond, surficial aquifer system 62SRP-S34 280241082230703 Yes 21.67 10.36

62nd St. Retention Pond 62SRP-POND 280242082230800 Yes 18.62 NA

Bonnie Brae Park, surficial aquifer system BBP-S45 280228082231501 Yes 16.22 13.72

Queensway Retention Pond QRP-S20 280311082223901 No 11 6.1

Jaqueline Arbor Retention Pond surficial well JARP-S40 280251082224201 No 13.22 11.28

Lynwood Park, Upper Floridan aquifer (upper) LP-H40 280303082230901 No 15.08 12.5

Lynwood Park, surficial aquifer system LP-S30 280303082230902 No 15.08 9.14

Lightfoot Recreation Center Retention Pond, Upper Floridan aquifer (lower) LRP-F160 280250082233001 Yes 20.24 48.77

Lightfoot Recreation Center Retention Pond, Upper Floridan aquifer (upper) LRP-H105 280250082233002 No 20.28 34.14

Lightfoot Recreation Center Retention Pond, surficial aquifer system LRP-S25 280250082233003 Yes 20.26 7.62

Lightfoot Recreation Center Retention Pond LRP-POND 280250082233200 Yes 18.64 NA
14 Simulations of Groundwater Flow and Particle Tracking Analysis ... to a Public-Supply Well near Tampa, FL, 2002-05

30

25

20

15

10

5

NAVD 88

5

10

15

20

0

METERS

WEST EAST

WP-S64

62SRP-S34

62SRP-H55

62SRP-F160 JARP-S40

MAS-R-S30

MAS-R-F64

MAS-R-F160

WP-F150

WP-F299

RP-S20

RP-F77

RP-F103

Hillsborough River

Greco Sink

WP-S64

GARC-S23

GARC-F75

GARC-F200

JARP-S40

Greco Sink

GARC-S23

GARC-F75

GARC-F200

SOUTH NORTH

113RC-S35 QRP-S20

113RC-H50

113RC-F190

THC-F75

BBP-S45

Paleo sink

TTP-4

(PSW)

*

* * *

* * * * *

(VERTICAL EXAGGERATION 52X)

(VERTICAL EXAGGERATION 54X)

0.25 0.5 0.75 1.0 KILOMETERS

METERS

METERS METERS

THC-F197

THC-SA/H46

SAND WITH INCREASING CLAY

CONTENT WITH DEPTH

SAND, CLAY, AND CARBONATE MUD

LIMESTONE

INFERRED HYDROGEOLOGIC CONTACT

GROUNDWATER FLOW DIRECTION

WELL SCREENS EXCEED THE SECTION DEPTH

WELL AND WELL IDENTIFIER

— Height of box is cased

and screen interval;

geologic control data

30

25

20

15

10

5

NAVD 88

5

10

15

20

30

25

20

15

10

5

NAVD 88

5

10

15

20

30

25

20

15

10

5

NAVD 88

5

10

15

20

0 0.25 0.5 0.75 1.0 MILES

0 0.25 0.5 0.75 1.0 KILOMETERS

0 0.25 0.5 0.75 1.0 MILES

EXPLANATION

A A'

B B'

Surficial

Aquifer

System

Upper Floridan Aquifer

Confining

Unit

Intermediate

Upper

Floridan

Aquifer

Surficial

Aquifer

System

Intermediate

Confining Unit

Figure 8. Hydrogeologic sections across the study area. Trace of sections is shown in figure 7. PSW is the selected public-supply well.
Methods 15

levels measured during sampling, was used for comparison 

to simulated water levels during groundwater flow model 

calibration. Groundwater levels and retention pond and riverstage data collected for this study are summarized in table 2 

and figure 9. Groundwater hydraulic gradients were calculated 

for measurements for wells open to the surficial aquifer 

system and intermediate confining unit or Upper Floridan 

aquifer (if the intermediate confining unit was not present) at 

well nests GARC, 113RC, RP, THC, and WP, and between 

the upper and the lower open intervals of the Upper Floridan 

aquifer at well nests GARC, LRP, MAS, 62SRP, RP, THC, 

and WP, and used in the parameter estimation process.

Development and Calibration of Local-Scale 

Groundwater Flow and Particle Tracking Model

A local-scale, steady-state, three-dimensional numerical 

groundwater flow model was developed and used in this 

study to evaluate the movement of water and solutes from 

recharge areas to the selected PSW. The model was nested 

within the regional model used for the TANC regional analysis 

(Crandall, 2007). The local-scale model was constructed 

and run using the USGS finite-difference groundwater flow 

model MODFLOW 2000 (Harbaugh and others, 2000). The 

parameter estimation feature of this code was activated to 

estimate hydraulic parameters and recharge properties using 

an automated nonlinear-regression approach. Flow paths and 

traveltimes associated with simulated particles that discharge 

to wells were determined using MODPATH version 4.3—the 

USGS particle tracking software (Pollack, 1994). Both the 

groundwater flow and the particle-tracking simulations were 

refined in concert to obtain the best match for simulated 

water levels and groundwater age tracer concentrations and 

to assure that hydraulic parameter estimates were reasonable. 

Later, the model was put into UCODE (Poeter and others, 

2005), which was used to refine the estimated hydraulic 

property values from MODFLOW-2000 as well as to estimate 

porosity values used by MODPATH. The UCODE_2005 is 

a parameter-estimation program that can be used with any 

program or combination of programs with numerical (ASCII 

or text only) input and output files. This program allowed for 

observations of water-level gradient and concentrations of the 

environmental tracers, tritium and sulfur hexafluoride, to be 

used in addition to water levels in estimating the parameters 

for MODFLOW and MODPATH.

Table 2. Summary of hourly water-level measurements obtained from monitoring wells, stormwater retention 

ponds, and the Hillsborough River in the local-study area, December 11, 2003 to November 17, 2005.

Short site 

name

Dates of measurement Number 

of 

days

Water-level measurements, in meters

Start date End date Mean Median Maximum Minimum 

BBP-S45 1/21/2005 11/14/2005 297 6.50 6.43 7.20 5.83

GARC-F200 1/28/2004 11/16/2005 659 6.76 6.62 8.44 5.76

GARC-S23 1/28/2004 11/16/2005 659 6.80 6.62 8.77 5.79

LRP-F160 1/21/2005 11/14/2005 298 6.48 6.41 7.11 5.82

LRP-POND 5/26/2005 11/7/2005 166 18.66 18.66 19.19 18.43

LRP-S25 1/21/2005 11/14/2005 297 17.32 17.26 18.03 16.66

62SRP-S34 6/10/2005 10/24/2005 135 12.64 12.74 13.12 11.96

62SRP-POND 5/31/2005 11/2/2005 112 18.27 18.27 19.67 17.61

HRIVER 12/12/2003 11/16/2005 704 6.58 6.62 7.07 5.90

MAS-R-F160 1/21/2005 11/14/2005 297 6.48 6.43 7.14 5.86

MAS-R-F64 1/21/2005 11/14/2005 296 7.17 7.00 7.97 6.66

MAS-POND 5/25/2005 11/14/2005 174 15.67 15.64 17.90 14.83

RP-F103 12/11/2003 11/17/2005 689 6.64 6.57 7.84 5.77

RP-F77 12/11/2003 11/17/2005 708 6.66 6.56 8.00 5.79

RP-S20 12/11/2003 11/17/2005 640 6.87 6.73 8.22 6.01

WP-F150 12/11/2003 11/16/2005 685 6.45 6.35 7.85 5.50

WP-F299 12/11/2003 11/16/2005 639 6.34 6.27 7.43 5.55

WP-S64 12/11/2003 11/16/2005 639 8.56 8.37 10.10 8.13
16 Simulations of Groundwater Flow and Particle Tracking Analysis ... to a Public-Supply Well near Tampa, FL, 2002-05

Groundwater Flow and Advective 

Tracking Simulation

The local-scale groundwater flow model simulates steadystate conditions in an aquifer system that is representative of 

long-term stresses, such as pumping and recharge. Pumping 

stresses used in the local-scale model are those from calendar 

year 2000. Pumping during calendar year 2000 was generally 

representative of pumping that has occurred in the area for 

the past 15 years or more, and pumping levels are unlikely 

to change significantly in the foreseeable future, because the 

area is already fully developed for residential and commercial 

uses. Also, concerns over declines in lake levels that began in 

the early 1990s, have prevented further pumping of the Upper 

Floridan aquifer for water supply. River stages are averages 

based on long-term gaging station data and regression between 

gages. Total recharge in the local-scale model area was based 

on parameter estimation but was checked against long-term 

estimates of recharge, which ranged from 12 to 65 cm/yr. 

The average total recharge in the model area is 33.8 cm/yr. 

Water levels used for comparison to model output are based 

on median water levels measured in the 28 wells installed for 

the TANC local-scale study from 2003 to 2005 and 12 wells 

operated by Temple Terrace and the Southwest Florida Water 

Management District.

Model Uncertainties and Limitations

The accuracy of the groundwater flow models is limited 

by the modeling software, assumptions made during model 

development, and results of model calibration and sensitivity 

analysis. A groundwater flow model is a means of portraying 

and testing a conceptual understanding of an aquifer system. 

DATE

11/1/03 3/1/04 7/1/04 11/1/04 3/1/05 7/1/05 11/1/05

22

20

18

16

14

12

10

8

6

4

TTP-4 (PSW)

RP-F103

RP-F77

RP-S20

WP-F150

WP-F299

WP-S64

GARC-F200

GARC-S23

BBP-S45

LRP-F160

MAS-R-F160

MAS-R-F64

62SRP-S34

LRP-POND

MAS-POND

62SRP-POND

MEAN DAILY WATER LEVELS, IN METERS

LRP-S25

Figure 9. Median daily water levels from continuous water-level recorders in monitoring wells, stormwater retention ponds, 

and the Hillsborough River station, and various isolated measurements made in the local-scale study area, December 11, 2003 

to November 17, 2005. PSW is the selected public-supply well.
Groundwater Flow and Advective Tracking Simulation 17

Because groundwater flow systems are inherently complex, 

simplifying assumptions are necessary in developing and 

applying these model codes (Anderson and Woessner, 1992). 

Groundwater flow models solve for average conditions within 

each cell. The parameters were interpolated or extrapolated 

from measurements and (or) estimated during calibration. 

As such, the intent in developing the groundwater flow model 

was not to reproduce every detail of the natural system, but 

rather to portray its dominant characteristics. This is especially 

true in aquifers with karst features where there is an extreme 

difference in aquifer properties at scales much smaller than the 

minimum representation in the numerical model.

Results from the local-scale model described herein 

should be interpreted generally and are best suited for 

comparative analysis at this location. However, the model can 

be used to calculate nitrate concentrations based on particle 

tracking and this can be used to indicate, in general terms, 

whether concentrations are likely to go up or down in the 

selected PSW.

It is assumed for a steady-state model that the system is 

in equilibrium. Although water-level hydrographs suggest this 

generally is the case in the local-scale model area over time, 

the data are not unequivocal. Errors related to this assumption could be notable, and consideration should be taken in 

interpreting model results and analyses that depend on model 

output, including particle tracking. The boundary conditions 

for the local-scale model are derived from the regional-scale 

model. To the extent that the regional model may contain 

errors in representing the flow system in the local-scale model 

domain, the boundary conditions used in the local-scale model 

also could be a source of model error.

Model Geometry and Discretization

The regional-scale model was used to provide boundary 

conditions, initial hydraulic heads, initial recharge estimates, 

drain locations, initial drain and river conductances, and 

pumping data for the local-scale model. Specified fluxes 

at the edges of the local-scale model were extracted from 

the regional model using methods developed by Leake and 

Claar (1999). The regional model is discussed by Crandall 

(2007), and updates are presented in the appendix of this 

report. The regional-scale groundwater flow model calibrated 

for the regional TANC study covers more than 5,400 km2 in 

the northern Tampa Bay region and consists of 4 layers, 227 

columns, and 234 rows with the cell size ranging from 200 

to 1,600 m on a side (fig. 10). The regional-scale model was 

updated using parameter estimation in MODFLOW-2000 

to gain better insight into recharge, to determine the most 

probable flow paths, and to improve estimates of boundary 

conditions and initial hydraulic properties. These updated 

hydraulic properties and recharge estimates were used for 

initial estimates in the local-scale model.

The local-scale model grid has a finer discretization than 

the regional-scale model and incorporates refined parameter 

zones and karst features. Hydraulic properties used in the 

updated regional-scale model were revised during local-scale 

model calibration. Like the regional-scale model, the localscale groundwater flow model grid is also rotated 50° counterclockwise from the north to simulate the anisotropy created 

by the secondary porosity features (the orientation of fracture 

planes) and to minimize flux errors along primary flow paths. 

Regionally, the predominant fracture orientation, identified 

using photolineament techniques by Williams (1985) and 

Culbreath (1988), is bimodally distributed with maxima at -50 

and +40°.

The local-scale model grid is discretized into 5,520 cells 

(80 rows and 69 columns) of 125 m on each cell side. The 

local-scale model has 13 layers representing 6 hydrogeologic 

units (fig. 4). Some model layers may represent the same 

hydrogeologic unit; for example, model layers 5 to 7 explicitly represent the upper part of the Tampa Member above 

the conduit layer. The upper and lower parts of the Tampa 

Member hydrogeologic unit was subdivided into multiple 

model layers for geochemical or particle tracking purposes to 

prevent problems with weak sinks. In these cases, the properties of the hydrogeologic unit remain unchanged throughout 

the multiple layers composing the unit.

Model layers 1 to 3 correspond to the surficial aquifer 

system (fig. 4). The top of this layer ranges from 0.83 to 

26.23 m above NAVD 1988; the bottom ranges from 0.83 

to 6.42 m above NAVD 1988. The thickness of this unit 

generally ranges from 0 to 21.78 m, but can be much greater 

where sinkholes are present because of surficial materials that 

collapse into the solution features. The median thickness of the 

three layers composing the surficial aquifer system is 5.36 m. 

This hydrogeologic unit was divided into three model layers 

to attempt to track changes in oxidation-reduction processes 

encountered along flow paths. Layer 1 is active predominately 

on the terraces and is inactive in the northern part of the model 

area in and along the Hillsborough River. Layer 2 is active in 

87 percent of the model-grid cells and is inactive in or along 

the Hillsborough River on the eastern side of the model area 

and along the westernmost Hillsborough River cells below the 

dam, along Cow House Creek, in drain cells on the western 

edge of the model area, and in the northern wetlands. Layer 3 

is active throughout most of the model area and is inactive in 

river cells below the dam and drain cells on the southeastern 

part of the model area. The three layers that correspond to the 

surficial aquifer system have identical hydraulic properties.

Layer 4 represents the intermediate confining unit. The 

bottom of this layer ranges from -0.48 m below to 5.42 m 

above NAVD 1988, and the median is 2.73 m above NAVD 

1988. The areal extent of layer 4 is identical to that described 

for model layer 3. The thickness of layer 4 is set to 1 m 

throughout the model area because, in some areas, the layer is 

either not present or very thin, which causes problems for 

solving the matrix. Layer 4 hydraulic properties alternate 
18 Simulations of Groundwater Flow and Particle Tracking Analysis ... to a Public-Supply Well near Tampa, FL, 2002-05

28°00’00’’

Local-Scale

Model Boundary

Simulated

depressions/

sinkholes

Simulated Conduit Layer 8

Local-Scale

Model Grid

TTP-4

(PSW)

82°45'

28°00'

28°15'

28°30' Gulf of Mexico

LOCAL-SCALE MODEL AREA

REGIONAL MODEL BOUNDARY

Albers Equal Area projection, units: meters, False Easting: 0, False Northing: 0

Central meridian: -83°00', 1st satndard parrallel: 29°30', 2nd standard parrallel: 45°30'

Latitude of origin: 23°00', Datum is NAD 1983

EXPLANATION

0 10 KILOMETERS

82°30' 82°15' 82°00'

CITY OF

TAMPA

CITY OF

TEMPLE

TERRACE

0 10 MILES

PASCO COUNTY

HERNANDO

COUNTY

SUMTER

COUNTY

POLK

COUNTY

HILLSBOROUGH COUNTY

PINELLAS

COUNTY

LAKE COUNTY

Tampa

Bay

82°22’30’’

28°00’00’’

82°22’30’’

EXPLANATION

Elevation, in meters

above NAVD 88

High : 27.78

Low : 0.18

(A)

(B)

0 1 2 3 4 KILOMETERS

0 1 2 MILES

Figure 10. Local-scale model area in relation to the regional-scale modeled area. PSW is the selected public-supply well.
Groundwater Flow and Advective Tracking Simulation 19

between those of the surficial aquifer system and intermediate 

confining unit, depending on whether or not a depression 

(sinkhole) has been identified in the particular model-grid cell.

Model layers 5 to 11 represent the Tampa Member in 

the Upper Floridan aquifer. For particle tracking purposes, 

this unit was subdivided into seven model layers to facilitate 

geochemical and flow path analysis. Layers 5, 6, and 7 represent the upper part of the Tampa Member and have identical 

hydraulic properties and thicknesses in corresponding model 

cells. The bottom of model layer 7 ranges from -18.26 to 

-29.72 m below NAVD 1988. The median total thickness of 

layers 5 to 7 is 28.11 m; however, the overall thickness ranges 

from 21.9 to 34.6 m. Layers 5, 6, and 7 thicken to the south 

within the Tampa Member. Model layer 8 represents the highly 

transmissive cavernous zone in the approximate middle of the 

Tampa Member. This layer was set to a constant thickness of 

3 m throughout the model—approximately based on caliper 

and video logs of the selected PSW. The bottom of layer 8 

ranges from -21.3 to -32.7 m below NAVD 1988. Layers 9, 

10, and 11 represent the lower part of the Tampa Member. The 

bottom of layers 9 to 11 ranges from -49.4 to -66.7 m below 

NAVD 1988. These three layers have identical hydraulic 

properties and thicknesses for each corresponding row/column 

address. The median total thickness of model layers 9 to 11 is 

34.1 m, but thicknesses range from 28.2 to 36.4 m.

Model layers 12 and 13 represent the Suwannee 

Limestone and (or) Ocala Limestone and possibly the Avon 

Park Formation in some places. The lower model boundary 

coincides with the 5,000-mg/L chloride concentration level. 

The lower extent of the freshwater flow system (Sepulveda, 

2002) at which the chloride boundary occurs ranges from 

-208.8 to -268.9 m below NAVD 1988. The total thickness of 

model layers 12 and 13 ranges from 145.1 to 210.0 m with a 

median thickness of 177.5 m. These model layers thin to the 

southwest.

Karst features are incorporated into model layers 1 to 11 

by using large vertical hydraulic properties, anisotropy, high 

recharge, and low porosity values where karst features are 

thought to be present. Locations and diameters of closed-basin 

depressions (sinkholes) were estimated by compiling available 

coverages of sinkholes and identifying closed-basin depressions from the digital elevation map. A total of 635 closedbasin depressions were identified in the local-scale model 

area. Preferential groundwater flow direction due to fracture 

orientation is simulated in the model by specifying anisotropy, 

which has the effect of decreasing the value of the hydraulic 

conductivity by using a multiplication factor between 0 and 

1.0 along column cell faces of the model grid for layers 5 to 13 

compared to that along row cell faces. Vertical anisotropy was 

also used in layers 1 to 3 to represent disruption of horizontally deposited layers and flow paths due to collapse features 

(sinkholes). The karstic nature of layer 8 was simulated in 

the local-scale model by using large values for the hydraulic 

properties and low values for the porosity of this layer. Little 

information exists for the model area about karst features 

below layer 8.

Boundary Conditions and Model Stresses

Boundary conditions for the local-scale model include 

specified fluxes entering or exiting the model area at the 

boundaries, and stresses include recharge from precipitation, discharge from pumping, leakage to or from rivers, and 

discharge to drains. In the local-scale model, specified fluxes 

are used for boundary conditions on all sides and layers of 

the model, except layers 1, 2, and 4. Water flowing across the 

boundary into these layers is assumed to be negligible.

Description of Lateral Boundaries

The local-scale model employs lateral non-zero fluxes 

across boundaries so that they do not have to extend to the 

natural hydrologic groundwater divides, thus allowing the 

focus of the modeling effort to remain on the ACR of the 

selected PSW. Specified non-zero flux boundaries are used on 

all sides of the model area for layers 3 to 13, except in layers 1, 

2, and 4. Layers 1 and 2 are not present on all boundaries, so 

all of the non-zero boundary flux to the surficial aquifer system 

layers was placed in layer 3. Layer 4 represents the thin, laterally and vertically discontinuous, low permeability, high clay 

content intermediate confining unit and is most likely incapable 

of significant fluxes across the model boundaries; therefore, 

a zero specified-flux boundary condition was imposed on the 

boundaries for this layer. All specified non-zero boundary 

fluxes were obtained from the updated regional model and 

translated to the local-scale model-grid cells.

Flow from the specified flux boundaries enters the model 

area primarily through the northwestern and northeastern 

boundaries and discharges from the model area primarily 

through the southeastern and southwestern boundaries. An 

appreciable amount of boundary flow entering and discharging 

from the model area is the result of the low hydraulic heads 

associated with drains near the southwestern corner of the 

model grid. The influx to the model area across lateral 

boundaries (table 3) was 24,297.15 m3/d (row 1—northeastern 

boundary) and 16,470.97 m3/d (column 1—northwestern 

boundary). Most of the water enters through model layers 8, 

12, and 13, and discharges from the local-scale model area 

through the southeastern (column 69) and southwestern 

(row 80) boundaries at -20,290.85 and -8,722.76 m3/d, 

respectively. Most of the water enters and exits the aquifer 

through layer 12 at inflows of 17,083.69 m3/d, and layer 13 at 

inflows of 11,535.8 m3/d in the lower part of the model flow 

system. However, 48.8 percent of the water discharging from 

the model area flows through layers 5 to 11, representing 

the upper part of the Upper Floridan aquifer in the Tampa 

Member. Net discharge occurs from layer 3 through the northeastern and northwestern boundaries in the local-scale model 

area. The net discharge, although small compared to other 

boundary fluxes, most likely occurs because wetlands, located 

directly outside of the northern corner of the local-scale model 
20 Simulations of Groundwater Flow and Particle Tracking Analysis ... to a Public-Supply Well near Tampa, FL, 2002-05

area, are in the regional-scale model and probably affect the 

potentiometric surface enough to slope it northward in this 

area and make the net inflow negative for layer 3.

Simulation of Recharge and Discharge

Recharge is simulated by applying it directly to the top 

face of the highest active grid cell (layers 1 to 5) of the localscale model area. Simulated recharge from the infiltration 

of precipitation averages 33.78 cm/yr throughout the model 

area (total volumetric recharge/total model area); recharge 

is increased in cells with depressions or sinkholes (fig. 10). 

Recharge is applied to layers 1 to 5 (fig. 11); although layer 2 

(68 percent by volume) and layer 3 (22 percent by volume) 

receive the most recharge. Recharge is applied to layer 1 (10 

percent of volume), mostly on the northwestern terrace. Less 

than 1 percent of the total recharge is applied to layers 4 and 

5, mainly along the river below the dam (layer 5), in the drains 

on the southeastern corner (layer 5), and near the center of the 

southeastern boundary (layer 4).

The Hillsborough River and its major tributaries (Cypress 

Creek, Cow House Creek, and the Tampa Bypass Canal) were 

simulated by the local-scale model using the MODFLOW River 

package (fig. 12). A total of 374 river cells were specified in 

the model grid. All river cells above the dam are set in layer 3, 

and river cells below the dam are set in layer 5. For each river 

cell, river stage and bottom elevations are extrapolated between 

upstream gaging station 02303330 (outside of the local-scale 

model area), gaging station 02304000, and gaging station 

02304500 (at the dam), using linear interpolation and available 

observations (figs. 2 and 11). River bottom elevations were 

set at a constant 3.0 m below stage values. For tributaries with 

no gaging stations, stage and river bottom elevations were 

estimated using the digital elevation model for land surface and 

constants based on measured stages and river bottom elevations 

at gaging stations that were available.

The MODFLOW Drain package was used to simulate 

water discharge from the Upper Floridan aquifer to wetland 

areas in the local-scale model area. A total of 538 drain cells 

were specified in layers 5, 6, and 7. Drain cells are located in the 

northern wetlands (layer 5), northeastern corner (layer 5), and 

southwestern corner of the model area (model layers 5, 6, and 7), 

as shown in figure 12. The elevation of drains was initially 

based approximately on land-surface elevation and stream stage, 

if available. Drain stages ranged from 3.96 to 6.50 m.

Pumping

The local-scale model area (fig. 13; table 4) has 77 

public-supply, industrial, and agricultural wells. Pumping rates 

during 2000 were used for the steady-state model. Daily total 

withdrawals from 24 public-supply wells and 53 industrial and 

agricultural wells were 25,408 m3/d. The average withdrawal 

rate for the selected PSW was 884 m3/d in 2000. The average 

Table 3. Summary of specified flux by model boundary and layer.

Layer

Specified flux by model boundary, in cubic meters per day

Northeast Southwest Northwest Southeast

3 -251.07 -44.32 -654.32 -237.65

5 673.67 -196.33 519.38 -1,221.60

6 673.67 -196.33 519.38 -1,221.60

7 673.67 -196.33 519.38 -1,221.60

8 3,422.51 -864.17 2,473.2 -5,237.04

9 673.67 -196.33 519.38 -1,221.60

10 673.67 -196.33 519.38 -1,221.60

11 673.67 -196.33 519.38 -1,221.60

12 8,347.99 -3,050.53 5,870.34 -3,750.73

13 8,735.70 -3,585.76 5,665.47 -3,735.83

Total: 24,297.15 -8,722.76 16,470.97 -20,290.85
Groundwater Flow and Advective Tracking Simulation 21

withdrawal rate for the agricultural and industrial wells of 

94 m3/d, was estimated by Sepulveda (2002) and was based 

on the permitted daily average withdrawal rate. A summary of 

withdrawal rates by well in the local-scale model area during 

2000 is shown in table 4.

Wells are simulated in the local-scale model using the 

multi-node well package for MODFLOW (Halford and Hanson, 

2002). The multi-node well package is used to allow the model 

to determine the relative contribution of flow from each model 

layer based on the head and model properties of those layers. 

The package also allows for flow to be tracked between specific 

model layers instead of simply being removed from the simulated aquifer system. Withdrawals are focused in layers 5 to 12. 

Layer 8, representing the highly transmissive cavernous zone, 

has 35 percent of the total withdrawals. Layer 12 represents 

another very productive part of the Upper Floridan aquifer 

with 33 percent of the total withdrawals. Most of the remaining 

withdrawals are distributed between layers 5 to 7 and layers 9 

to 11. Withdrawals are less than 2 percent for each of the total 

volumes in layers 3, 4, and 13.

TTP-4 (PSW)

28°00’

82°22’30’’

28°00’

82°22’30’’

25.57

>25.57-44.68

>44.68-102.66

>102.66-102.66

>102.66-1610.14

EXPLANATION

Recharge, in centimeters per year

Streams, wetlands, and lakes

TTP-4 (PSW)

0 1 2 3 4 KILOMETERS

0 1 2 MILES

Figure 11. Distribution of recharge in the local-scale study area. PSW is the selected public-supply well.
22 Simulations of Groundwater Flow and Particle Tracking Analysis ... to a Public-Supply Well near Tampa, FL, 2002-05

Hydraulic and Other Estimated Parameters

Initial estimates for hydraulic parameters and other values 

in the local-scale model were derived from the calibrated 

regional model. However, parameters continued to be updated 

during local-scale model calibration. Initially, trial-and-error 

methods were used to adjust parameters between model runs, 

but eventually the model was put into MODFLOW-2000 and 

finally UCODE to obtain final values for hydraulic parameters, recharge, riverbed and drain conductances, and effective 

porosities to maximize the observations (including age tracer 

information). Eventually, 24 hydraulic, recharge, riverbed 

and drain conductances, and transport (effective porosities) 

parameters were used to represent the groundwater flow 

system in the local-scale model. Finally, 17 parameters were 

able to be estimated using parameter estimation techniques. 

Parameter estimates were resolved for recharge and hydraulic 

conductivities representing the surficial aquifer system and 

Upper Floridan aquifer and porosities using UCODE (Poeter 

and others, 2005) and observations of water levels, water-level 

River cells

River gain

River loss

Drain cells

Sinkhole/depression

Streams, wetlands, and

lakes

TTP-4 (PSW)

TTP-4

(PSW)

EXPLANATION

28°00’

82°22’30’’

82°22’30’’

28°00’

0 1 2 3 4 KILOMETERS

0 1 2 MILES

Figure 12. River and drain cells in the local-scale model and study areas. TTP-4 is the selected public-supply well.
Groundwater Flow and Advective Tracking Simulation 23

gradients, and age tracer concentrations. Values for riverbed 

and drain conductances and horizontal hydraulic conductivity 

for layer 4, representing the intermediate confining unit, were 

not estimated due to lack of observations.

Generally, hydraulic conductivity parameters in each 

model layer were conceptualized as a combination of a “base” 

hydraulic conductivity or recharge value for the porous block 

with anisotropy applied to vertical and horizontal values in the 

intermediate confining unit and Upper Floridan aquifer and 

an adjusted value that applies only to the areas where karst 

features have been identified (table 5). For model layers 1 

to 3 representing the surficial aquifer system and recharge 

parameters, multipliers are applied to the hydraulic parameters 

that are based on the percentage of cell encompassed by one 

or more closed-basin depressions. For hydraulic parameters in 

layers 4 to 13 (excluding layer 8) representing the intermediate 

confining unit and Upper Floridan aquifer, anisotropy and a 

new greater hydraulic value is usually applied to cells in layers 

that include a closed-basin depression. Layer 8 has constant 

and hydraulic properties.

Local-scale model

boundary

Pumping wells–Number

refers to table 4

Gaging station and

number

EXPLANATION

9

8

7

6

5 4 3

2

1

77 76

74 75

72 73

70

68 69

67 66

65 64

63

61

60

59 58

57

56 55 54 53

52 51 50 49 48 47 46 45

42 43 44

41

40

38

37 34

33

32 31 30 29 28

27 26

25

24

23

19

18

16 15

10

02304500

02304040

28°00’

82°22’30’’

82°22’30’’

28°00’

25

02304500

0 1 2 3 4 KILOMETERS

0 1 2 MILES

Figure 13. Location of groundwater withdrawal wells and gaging stations in the local-scale study area.
24 Simulations of Groundwater Flow and Particle Tracking Analysis ... to a Public-Supply Well near Tampa, FL, 2002-05

Table 4. Average groundwater withdrawal rates from public-supply wells (wells 1–24), industrial wells, and 

agricultural wells in the local-scale study area during 2000.

[Locations of wells are shown in fig. 13]

Well 

number

Elevation of 

top of open 

interval 

(meters)

Elevation 

of bottom of 

open interval 

(meters)

Withdrawal 

rate (cubic 

meters per 

day)

Well 

number

Elevation of 

top of open 

interval 

(meters)

Elevation 

of bottom of 

open interval 

(meters)

Withdrawal 

rate (cubic 

meters 

per day)

1 6.83 -47.61 261 40 4.79 -25.69 136

2 5.98 -78.95 261 41 5.69 -24.79 57

3 5.07 -31.08 110 42 6.34 -24.14 254

4 -21.38 -31.13 17 43 8.61 -21.87 77

5 -34.33 -80.05 240 44 8.61 -21.87 77

6 -18.81 -140.73 132 45 7.20 -23.28 27

7 -33.50 -146.27 22 46 6.43 -24.05 27

8 6.51 -74.40 32 47 6.95 -23.53 27

9 1.80 -23.92 95 48 8.12 -22.36 98

10 -18.18 -109.62 238 49 5.86 -24.62 27

11 -9.07 -23.39 3,780 50 6.65 -23.83 27

12 -13.74 -25.01 3,901 51 5.43 -25.05 40

13 -28.74 -101.90 1,739 52 5.45 -25.03 40

14 -12.40 -29.47 680 53 4.76 -25.72 353

16 -11.46 -28.22 3,591 54 4.77 -25.71 353

17 -24.35 -126.76 267 55 4.68 -25.80 353

18 -21.34 -126.50 267 56 4.72 -25.76 353

19 -24.27 -125.77 267 57 6.58 -23.90 27

20 -8.61 -126.56 1,701 58 4.68 -25.80 4

21 -5.12 -127.04 1,323 59 7.75 -22.73 27

22 4.22 -100.93 1,134 60 5.87 -24.61 27

23 .12 -77.61 49 61 5.81 -24.67 27

24 5.24 -99.01 340 62 5.87 -24.61 27

25 6.65 -23.83 301 63 5.74 -24.74 23

26 6.56 -23.92 301 64 5.64 -24.84 27

27 6.62 -23.86 261 65 5.37 -25.11 30

28 6.15 -24.33 49 66 6.01 -24.47 27

29 5.52 -24.96 19 67 6.28 -24.20 68

30 5.61 -24.87 19 68 5.49 -24.99 42

31 6.88 -23.60 87 69 6.32 -24.16 23

32 5.17 -25.31 34 70 5.59 -24.89 15

33 6.26 -24.22 121 71 5.66 -24.82 85

34 6.25 -24.23 121 72 5.55 -24.93 8

35 6.29 -24.19 121 73 5.64 -24.84 85

36 5.78 -24.70 125 74 5.62 -24.86 53

37 6.50 -23.98 108 75 5.66 -24.82 33

38 6.51 -23.97 108 76 5.69 -24.79 33

39 5.74 -24.74 125 77 4.81 -25.67 45
Groundwater Flow and Advective Tracking Simulation 25

Table 5. Hydraulic, recharge, and transport parameters by hydrogeologic unit and model layer used in local-scale groundwater flow and 

particle-tracking simulations.

[Hydraulic conductivity and recharge, in meters per day; drain and river conductance, in cubic meters per day per meter, cm/yr, centimeters per year; 

HK, hydraulic conductivity; VK, vertical hydraulic conductivity; HANI, horizontal anistropy; VANI, vertical anistropy]

Model 

layer

Parameter 

type

Base 

value

without

sinkholes

Base value in 

depression/karst 

areas

Description

Surficial Aquifer System

 Layers 1–3 HK 36.78 0.063–36.78 The HK for the surficial aquifer system. The HK in the closed basin depressions 

varies and is calculated as a percent of cell in the depression basin. The values are 

considerably lower than non-depression areas, which probably is due to the altered 

structure of the surficial aquifer system compared to undisturbed areas

VK 0.3678 0.063 The VK for the surficial aquifer system is calculated as HK divided by the VANI. 

In areas of depressions, VANI is 1.0. Outside of depressions the VANI is 100.0 

Effective 

porosity

0.322 0.322 Porosity for the surficial aquifer system

Intermediate Confining Unit

 Layer 4 HK 0.0103 0.0103 The HK for the intermediate confining unit

VK 4.5x10-5 541 The VK for the intermediate confining unit. In areas with depressions and along the 

river and drains, the higher value is used; otherwise the lower value is used

Effective 

porosity

0.011 0.004 The estimated number is lower than expected probably due to preferential flow paths

Upper Floridan Aquifer

 Layers 5–7 HK 36.96 (rows), 

15.63 (columns)

36.96 (rows), 

15.63 (columns)

The HK for the upper part of the Upper Floridan aquifer. Because anisotropy is 

used, there is one conductivity along rows (northwest to southeast) and another 

along columns. The hydraulic conductivity along columns is calculated as (HANI = 

0.423)*HK along rows

VK 0.0424 541 The VK for the Upper Floridan aquifer. The VK uses the high value in areas of 

depressions, drains, and the river and uses the base value everywhere else

Effective 

porosity

0.12 0.004 Effective porosity of the Upper Floridan aquifer. If a depression is present, the lower 

value is used; otherwise 0.12 is used

 Layer 8 HK 674 (rows), 

74.95 (columns)

674 (rows), 

74.95 (columns)

The HK for the Upper Floridan aquifer in layer 8. Because anisotropy is used, this 

is the conductivity along rows (northwest to southeast). The HK along columns is 

calculated as the HANI * HK(rows) = HK(columns); (HANI = 0.1105)

VK 541 541 The VK for layer 8

Effective 

porosity

0.004 0.004 Porosity for layer 8. The low value would indicate that the actual volume in which 

the conduit flow takes place is small

 Layers 9–13 HK 36.96 (row), 

15.63 (column)

36.96 (row), 

15.63 (column)

The HK for the lower part of the modeled Upper Floridan aquifer. Because 

anisotropy is used, there is one conductivity along rows and another along columns 

(HANI = 0.423)

VK 0.0424 456 The VK for the lower part of the modeled Upper Floridan aquifer. The VK uses a 

higher value (541 meter per day) where there are drain cells on the western edge of 

the modeled area. The model uses the lower value elsewhere 

Effective 

porosity

0.12 0.12 Porosity of the porous block in the lower modeled Upper Floridan aquifer
26 Simulations of Groundwater Flow and Particle Tracking Analysis ... to a Public-Supply Well near Tampa, FL, 2002-05

Hydraulic Parameters 

The horizontal hydraulic conductivity for model layers 

1 to 3, representing the surficial aquifer system, ranges from 

0.06 to 36.78 m/d (fig. 14; table 5). The values for individual 

cells are derived by adding together two horizontal hydraulic 

conductivity terms: one for cells with sinkholes and one for 

cells without sinkholes. Each conductivity value is prorated 

by the percentage of the cell with undisturbed or disturbed 

(sinkholes) porous media. The base horizontal hydraulic 

conductivity value for undisturbed porous media is 36.78 m/d, 

whereas the base horizontal hydraulic conductivity value for 

cells that are entirely encompassed by sinkholes is 0.063 m/d. 

For example, a model cell entirely encompassed by a depression has a horizontal hydraulic conductivity of 0.06 m/d; a cell 

entirely without sinkholes has a hydraulic conductivity value 

of 36.78 m/d. A cell that is 50-percent sinkhole and 50-percent 

undisturbed porous media would have a horizontal hydraulic 

conductivity of 18.42 m/d. Horizontal anisotropy was not 

specified for layers 1 to 3, representing the surficial aquifer 

system, because presumably there is no preferential direction 

of flow in this relatively homogenous sand-and-gravel aquifer.

The vertical hydraulic conductivity for layers 1 to 3 

varies inversely with the horizontal hydraulic conductivity 

(vertical hydraulic conductivity equals horizontal hydraulic 

conductivity divided by the vertical anisotropy value). The 

horizontal hydraulic conductivity is divided by 100, so that 

it equals 0.37 m/d in cells without sinkholes. The vertical 

hydraulic conductivity is 0.063/1.0 m/d or 0.063 m/d in cells 

with sinkholes. Vertical hydraulic conductivity values range 

from 0.063 to 0.37 m/d, increasing in undisturbed sediments. 

This may reflect disturbed beds and disruption to transport 

through these layers.

The horizontal hydraulic conductivity for layer 4, 

representing the intermediate confining unit, is either 0.0103 

or 1.03 m/d. The base value for undisturbed porous media 

is 0.0103 m/d, but where sinkholes are present (fig. 14), the 

hydraulic conductivity is multiplied by 100. The base vertical 

hydraulic conductivity for the intermediate confining unit is 

4.5 x 10-5 m/d derived in the local-scale model calibration. 

However, in depression areas and under the river and drains, 

this value, derived in UCODE, is specified at 541 m/d to simulate the rapid transmission of water through short-circuiting 

mechanisms (sinkholes). Anisotropy was not estimated for this 

layer due to the sparse number of observations and insensitivity of the parameters associated with layer 4.

The base horizontal hydraulic conductivity for layers 5 

through 13 (except for layer 8), representing the Upper 

Floridan aquifer, is 15.63 m/d along columns and 36.96 m/d 

along rows (table 5). The horizontal hydraulic conductivity 

anisotropy was estimated in UCODE as 0.423. This strong 

tendency toward anisotropy controls flow path lines and shifts 

much of the simulated flow along rows (northwest to southeast) 

in the model grid, which is consistent with previous studies 

and photolineament work (Langevin, 1998). Vertical hydraulic 

Model 

layer

Parameter 

type

Base 

value

without

sinkholes

Base value in 

depression/karst 

areas

Description

Other Hydraulic and Stress Properties 

 Layers 1–5 Recharge 0.0007 

(or 25.6 cm/yr)

0.00052

(or 19 cm/yr

karst multiplier)

The base recharge (25.6 cm/yr) is applied everywhere; however, in areas of 

depressions, this base value is increased by a value (19 cm/yr) multiplied by a 

percentage based on the area of the depression that contributes recharge to the cell. 

This number is added to the base recharge

Drain and Riverbed Conductance Parameters and Values

 Layers 5–7 Drain 

conductance

0.0010–0.000010 0.001–100,000 Parameters factor multipliers—0.001 used for the northern wetlands—conductances 

from regional model multiplied by the parameter value. Parameter values are 

1,000 for the northeastern drains, and 100,000 for the southwestern drains to force 

boundary flow into agreement with the regional flow model

 Layers 3, 5 Riverbed 

conductance

39.6 39.6–2,185.7 River cells in layers 3 above the dam and in layer 5 below the dam. Riverbed 

conductances from regional model multiplied by this constant factor multiplier

Table 5. (Continued) Hydraulic, recharge, and transport parameters by hydrogeologic unit and model layer used in local-scale groundwater 

flow and particle-tracking simulations.

[Hydraulic conductivity and recharge, in meters per day; drain and river conductance, in cubic meters per day per meter, cm/yr, centimeters per year; 

HK, hydraulic conductivity; VK, vertical hydraulic conductivity; HANI, horizontal anistropy; VANI, vertical anistropy]
Groundwater Flow and Advective Tracking Simulation 27

conductivity is specified at 0.0424 m/d in the porous block 

and 541 m/d in sinkholes, along the river, and under drains in 

layers 5 to 7. In layers 9 to 13, the vertical hydraulic conductivity is specified at 0.0424 m/d everywhere, except under 

the western drain where the vertical hydraulic conductivity 

is 541 m/d. In layers 5 to 7 representing the top of the Tampa 

Member, there is no adjustment for sinkholes—only the row/

column anisotropy.

Layer 8 represents the highly transmissive cavernous 

layer in the approximate middle of the Tampa Member 

(the top unit of the Upper Floridan aquifer). The horizontal 

hydraulic conductivity of this layer is 674 m/d along the rows 

and 74.95 m/d along the columns (table 5). The value for 

the horizontal hydraulic conductivity anisotropy resolved in 

UCODE is 0.1105 (table 5). The vertical hydraulic conductivity 

also resolved in UCODE is 541 m/d throughout the layer.

0.06-2.89

2.89-7.93

7.93-12.62

12.62-17.03

17.03-21.38

EXPLANATION

Horizontal hydraulic conductivity,

in meters per day

Sinkhole/depression

Streams, wetlands, and lakes

TTP-4 (PSW)

21.38-25.34

25.34-29.16

29.16-32.62

32.62-35.39

35.39-36.78

28°00’

82°22’30’’

82°22’30’’

28°00’

TTP-4

(PSW)

0 1 2 3 4 KILOMETERS

0 1 2 MILES

Figure 14. Distribution of horizontal hydraulic conductivity in model layers 1 to 3, representing the surficial aquifer 

system. PSW is the selected public-supply well.
28 Simulations of Groundwater Flow and Particle Tracking Analysis ... to a Public-Supply Well near Tampa, FL, 2002-05

Four additional hydraulic parameters, one for riverbed 

conductance and three for drain conductance (table 5), were 

simulated but not estimated with UCODE due to a lack 

of observations. Initial riverbed conductances were taken 

from the regional-scale model, but were then multiplied by 

a riverbed conductance parameter to increase the values for 

the local-scale model. The riverbed conductance parameter 

multiplier is specified at 39.6 for all model river reaches. Final 

riverbed conductances ranged from 39.6 to 2,185.7 m3/d per 

meter with a median value of 524.6 m3/d per meter. Drain 

conductances varied for each drain area. One drain parameter 

multiplier was specified for the central-north wetlands area 

(0.001). The net result of this parameter multiplier is that the 

drain conductance varies from 0.001 to 15.63 m3/d per meter 

in this drain area. The other two drain parameters are specified 

constants of 1,000 for the northeastern drains area and 100,000 

for the southwestern drains area in the local-scale model. 

These drain conductances originated from the regional-scale 

model and need to be set high to match observed hydraulic 

heads in the local-scale and regional-scale models.

Recharge Parameters

Recharge is simulated as the sum of two parameters 

values, and both were estimated using UCODE. The first 

recharge parameter represents an areal recharge value 

(0.0007 m/d—25.6 cm/yr) applied to all cells, representing 

a base areal recharge value that would be expected to occur 

through infiltration of precipitation throughout the localscale model area (table 5). The second recharge parameter 

represents an additional or focused recharge value that would 

be expected to occur in cells that contain closed-basin depressions. The second recharge parameter is a combination of 

a constant (0.00052 m/d—18.99 cm/yr) multiplied by the 

percentage of the ACR located in the closed-basin depression. Recharge is increased in cells that contain closed-basin 

depressions to try to mimic the focusing effects of sinkholes 

on recharge, because cells with sinkholes may drain a much 

larger area than one cell. Drainage basins for sinkholes can 

be very large. For example, if a sinkhole drains a 16-cell area, 

the recharge rate would be about 100 cm/yr for the entire 

sinkhole drainage basin. Final recharge totals for each cell 

is given by the sum of these two recharge parameters. Total 

recharge values for each model-grid cell ranged from 25.67 to 

1,610.14 cm/yr per cell (fig. 11). Most of the model-grid cells 

(95 percent) had a total recharge value of 25.57 cm/yr (base 

areal value). Only 55 model-grid cells (less than 1 percent) 

had total recharge values greater than 278.08 cm/yr.

Other Parameters

Model parameters controlling velocities and, therefore, 

particle-tracking age and tracer concentration results from 

the local-scale model include effective porosities for the 

groundwater flow system. Initial effective porosity values 

were based on literature values obtained in Knochenmus and 

Robinson (1996) and Langevin (1998), but were updated using 

the measured age tracer concentrations (sulfur hexafluoride 

and tritium) at the monitoring wells and the selected PSW as 

observations in the nonlinear regression using UCODE.

Final effective porosities were specified for each active 

cell according to the layer/aquifer/karst feature defined for that 

cell. Generally, the final effective estimated porosities used in 

this modeling effort are at the low end of a range of porosity 

values given in the literature (Domenico and Schwartz, 1990). 

A base effective porosity of 0.322 is used throughout layers 1 

to 3, representing the surficial aquifer system (table 5). The 

base effective porosity for layer 4, representing the intermediate confining unit, is 0.011; however, a value of 0.004 is 

used in the depressions. The calibrated base effective porosity 

for layer 4 is lower than expected based on values of about 

0.30 for clays in Domenica and Schwartz (1990), and may 

suggest that preferential flow paths are the main mechanism 

of flow through the clay layer. The base effective porosity for 

layer 5 through 13 (except for layer 8), representing the Upper 

Floridan aquifer, is 0.12; however, a value of 0.004 is used 

in layers 5 to 7 where there are depressions and throughout 

layer 8. Similar to the porosity in layer 4, the low value of 

0.004 suggests that the flow occurs only in a small percentage 

of the total volume of the aquifer, which is consistent with 

the concept of flow through conduits. All flow paths for the 

selected PSW and monitoring wells are within the areal extent 

of the local-scale model.

Parameter Sensitivity

The number and location of observations used in the 

model calibration affect parameter sensitivities and are directly 

responsible for which parameters can be estimated using 

parameter estimation (UCODE; Poeter and others, 2005). The 

two recharge parameters at 97.4 and 38.7 percent are the most 

sensitive parameters in the simulation (fig. 15). Other sensitive 

parameters are horizontal hydraulic conductivity and anisotropy of the Upper Floridan aquifer at 16.5 and 12.7, respectively. Other somewhat sensitive parameters include horizontal 

hydraulic conductivity and anisotropy for layers 5 to 13 

(excluding layer 8), riverbed conductance, effective porosity, 

and the vertical hydraulic conductivity for layer 4. Other 

parameters with sensitivities less than 10 percent include the 

hydraulic conductivity and anisotropy for layer 8 (highly 

transmissive cavernous zone in the Upper Floridan aquifer), 

anisotropy for layer 4 (intermediate confining unit), effective 

porosity and drain parameters, and hydraulic conductivity 

pertaining to layers 1 to 3 (surficial aquifer system).
Simulation of Particle Tracking 29

Simulation of Particle Tracking

Particle tracking was used to compute flow paths and 

advective traveltimes throughout the model area and to delineate the ACR and zone of contribution for the selected PSW. 

The ACR to a discharging well is defined as the area at the 

top of the water table where recharging water that enters the 

groundwater flow system will eventually be removed by the 

well. The zone of contribution is defined as the specific volume 

of aquifer material bounded by outermost flow paths that will 

recharge a discharging well (Focazio and others, 2002). These 

results were used to characterize the sources of recharge water 

reaching the well. Steady-state transport was assumed and 

recharge rates for individual particles of water entering the well 

were estimated from the simulated traveltimes. The recharge 

rates were combined with estimated input concentrations of 

several tracers (tritium, sulfur hexafluoride, and nitrate) to 

calculate flux-weighted concentrations of the tracers reaching 

the well. The resulting calculated concentrations were compared 

with measured concentrations to evaluate the adequacy of the 

model to simulate advective transport and, thereby, address 

several project objectives. Such objectives include: (1) assessing 

the effects of natural factors and human activities on contaminant occurrence in PSWs, and (2) providing information to 

managers so they can develop more effective strategies to deal 

with contamination problems affecting selected PSWs.

Particle tracking was performed using MODPATH, which 

uses cellular volumetric fluxes from a groundwater flow model 

and effective porosity to calculate linear velocity. The x, y, and 

z components of linear velocity are used to generate a velocity 

field using simple linear interpolation of the components 

between adjacent cell faces of the finite-difference cells. 

For steady-state simulations, an analytical expression for the 

flow path in each cell is calculated by direct integration of the 

velocity components. The traveltime calculated by this method 

represents an average traveltime for the advection of a “particle” 

of water or a conservative solute. Dispersion, diffusion, adsorption, retardation, degradation or any other transport process 

affecting the concentrations of contaminant constituents are not 

included in the MODPATH simulation for determining traveltimes of water. The simulated ages of water particles at a well 

are computed by tracing water particles either backward from 

the monitoring well toward the ACR or forward from the ACR 

to the open-interval of the selected PSW.

To calculate concentrations of tracers for parameter 

estimation, particle tracking was used to obtain a distribution 

of particle ages that were then associated with known input 

functions. A backward-tracking approach was used to obtain 

the age distributions. For the monitoring wells, 10 particles 

were evenly distributed in a vertical line over the open interval 

of a monitoring well. Particles were tracked backward toward 

the area of recharge. For the selected PSW, 100 particles, 

distributed on each cell face, were tracked using the backward 

particle tracking mode.

Each particle is associated with an end point and final 

traveltime, a flow pathline, and traveltime along the flow 

pathline. In addition,, the ACR and the zone of contribution 

are mapped from the resultant collection of flow pathlines 

and intersected with land-use coverages to identify current 

and potential sources of contaminants to the wells. Statistical 

information about land use in each ACR and the zone of contribution was computed and is described by Crandall (2007).

Figure 15. Composite scaled sensitivity 

of the parameters used to calibrate the 

groundwater flow and transport model 

in UCODE.

Focused recharge (sinkhole basin)

Base areal recharge

Base horizontal hydraulic conductivity for layers 5-7 and 9-13

Horizontal anisotropy layers 5-7 and 9-13

Riverbed conductance

Base porosity for layers 5-7 and 9-13

Vertical hydraulic conductivity for layer 4

Horizontal hydraulic conductivity for layer 8

Vertical conductivity for layers 9-13

Anisotropy layer 8

Horizontal anisotropy layer 4

Porosity for layers 1-3

Drain conductance (north wetlands)

Base horizontal hydraulic conductivity for layers 1-3

Drain conductance (southeastern)

Horizontal hydraulic conductivity for layer 4

Base vertical anisotropy for layers 1-3

Vertical hydraulic conductivity layers 4-8 (sinkholes)

Horizontal hydraulic conductivity layers 1-3 (sinkholes)

Vertical anisotropy layers 1-3 (sinkholes)

Base porosity layer 4

Porosity (sinkholes) layers 4-8

0 20 40 60 80 100

COMPOSITE SCALED SENSITIVITY, IN PERCENT
30 Simulations of Groundwater Flow and Particle Tracking Analysis ... to a Public-Supply Well near Tampa, FL, 2002-05

Calibration

Groundwater flow model and particle tracking calibration 

was achieved using 53 groundwater levels and gradient observations and 19 sulfur hexafluoride concentrations. Overall, 24 

parameters were used to define the simulation, including those 

for horizontal and vertical hydraulic conductivity, anisotropy, 

recharge, riverbed and drain conductance, effective porosity 

using UCODE (Poeter and others, 2005), a universal code 

for sensitivity, calibration (using parameter estimation), and 

uncertainty evaluations in concert with MODFLOW and 

MODPATH (Pollack, 1994). Only 17 of 24 parameters could 

be estimated in parameter estimation, which ultimately was 

due to insensitivity of the parameters (lack of critical observations). Estimated parameter values minimize the sum of 

squares differences between observed and simulated values to 

result in the best match between measured and simulated water 

levels, water-level gradients, and age tracer concentrations. 

No river leakage was used to calibrate the model, because 

all of the streamflow data for the Hilllsborough River was 

from the period before the construction of the Hillsborough 

River Reservoir Dam. In addition, there were no drain leakage 

observations to help calibrate the drain parameters.

The average weighted residual for all observations is 

-0.261. Some minor bias in the model exists as 38 residuals 

are greater than or equal to observed values and 53 residuals 

are less than observed values. Model residuals are smallest for 

water level and water-level gradients. There was a -0.01 percent discrepancy in the overall water budget (1.79199 m3

/s). 

Age-tracer concentrations matched well enough to suggest a 

reasonably good representation of the porosity and hydraulic 

parameters. Water levels, water-level gradients, flow patterns, 

and water budget components were used in the calibration of 

the groundwater flow model. These parameters are discussed 

in detail in the subsequent sections.

Water Levels and Water-Level Gradients

The groundwater flow model was calibrated using 42 

water-level observations. Water-level data came from various 

sources including TANC, City of Temple Terrace, and the 

Southwest Florida Water Management District (table 6). 

There were 28 water-level measurements from wells open 

to the Upper Floridan aquifer, 2 water-level measurements 

from wells open to the intermediate confining unit, and 12 

water-level measurements from wells open to the surficial 

aquifer system. Water-level gradients also were used to help 

calibrate the model; 7 were between the upper and lower 

parts of the Upper Floridan aquifer, and 5 were between the 

surficial aquifer system and Upper Floridan aquifer. Waterlevel gradient observations were computed from wells in well 

nest: GARC, LRP, MAS, 62SRP, RP, THC, and WP. Weights 

used for water levels and gradients ranged from 0.20 to 2.00 

(table 6).

Nonweighted simulated hydraulic heads and hydraulic 

head gradients open to layers 1 to 3 (representing the surficial 

aquifer system) and one well open to layer 4 (representing the 

intermediate confining unit) matched the observed groundwater level and gradient measurements slightly worse than 

wells open to layers 5 to 13, representing the Upper Floridan 

aquifer (table 6; figs. 16, 17). The surficial aquifer system 

residuals ranged from -1.91 to 3.57 m (table 6) and had a 

median residual of -0.28 m; the Upper Floridan aquifer residuals ranged from -1.80 to 1.71 m and had a median residual of 

0.04 m. Observed water levels matched the simulated water 

levels best for Upper Floridan aquifer well 113RC‐F190 

(residual of less than 0.01 m) and in surficial aquifer system 

wells at LP-S30 and 113RC-S35. The simulated water levels 

were 7.25 m in 113RC-H50m, the only well in the intermediate confining unit with a continuous recorder; observed 

water level was 7.37 m. The residuals of water-level gradients 

from the surficial aquifer system to the Upper Floridan 

aquifer ranged from -0.87 to -0.17 m; all of the simulated and 

weighted water-level gradients were slightly larger than the 

nonweighted residuals or observed values. Median water-level 

residuals in the local-scale TANC observation wells open to 

the surficial aquifer system or the intermediate confining unit 

were approximately balanced between simulated values that 

were lower and higher than observed values.

Simulated hydraulic head and hydraulic head gradient 

values from wells open to the Upper Floridan aquifer matched 

the observed water levels reasonably well (table 6; fig. 18). 

Simulated water levels in wells open to the Upper Floridan 

aquifer ranged from 4.85 to 8.6 m, whereas observed water 

levels ranged from 4.43 to 9.8 m. In the middle of the model 

area where most of the observation wells are located, the 

model fit improved (fig. 18). For example, well 113RC‐F190 

had a residual water level of less than 0.01 m. Well WP‐F299, 

the deepest monitoring well in the Upper Floridan aquifer 

installed for the TANC study, had a simulated water level of 

6.3 m and an observed water level of 6.19 m. The simulated 

median water levels in 13 wells open to the Upper Floridan 

aquifer were greater than the observed median water levels 

and less than the observed water levels in 14 wells. Waterlevel residuals ranged from -1.8 to 1.71 m and the median 

weighted residual was 0.05 m.

Flow Patterns

The simulated potentiometric surface of the water table, 

which in most areas is composed of part of the surficial 

aquifer system but in lower lying areas is composed of the 

Upper Floridan aquifer, was generally representative of the 

measured water-table surface in the local-scale model area. 

Water flowed from the northern corner of the model area 

toward the south and southeast. The simulated water-table 

surface was highest in layers 1 to 3 (8.5 m) in the northeastern 

corner of the model area boundary and the northwestern side 

of the model area on the terrace (fig. 19A). Most of layer 1 
Calibration 31

Table 6. Measured and simulated hydraulic heads and hydraulic head gradients, in the local-scale model, and 

residual, weighted residual, aquifer, and data source for monitoring wells.

[Hydraulic heads, in meters. TANC, Transport of Anthropogenic and Natural Contaminants; SWFWMD, Southwest 

Florida Water Management District; CTT, City of Temple Terrace. PSW is the selected public-supply well] 

Observation 

name

Observed 

value

Simulated 

value Residual Weighted 

residual

Data 

source

Hydraulic head—Upper Floridan aquifer

GARC-F200 6.63 6.74 -0.12 -0.16 TANC

GARC-F75 6.23 6.85 -.62 -.88 TANC

LRP-F160 7.4 5.79 1.61 2.28 TANC

LRP-H105 7.41 6.29 1.12 1.58 TANC

MAS-R-F160 7.44 5.73 1.71 2.42 TANC

MAS-R-F64 6.78 5.66 1.12 1.58 TANC

113RC-F190 6.85 6.84 .00763 .0108 TANC

113RC-H50 7.02 6.15 .88 1.24 TANC

62SRP-H55 8.1 7.29 .81 1.14 TANC

RP-F103 6.56 6.7 -.15 -.22 TANC

RP-F77 6.56 6.71 -.14 -.2 TANC

TTP-4 (PSW) 6.73 5.47 1.26 1.79 TANC

THC-F197 6.81 6.55 .25 .36 TANC

THC-F75 6.82 6.63 .19 .26 TANC

WP-F299 6.19 6.3 -.11 -.15 TANC

WP-F150 6.24 5.73 .5 .71 TANC

CHOOL_WELL_1 9.8 8.29 1.51 2.13 SWFWMD

DGE_17_DEEP 9.43 8.6 .83 1.18 SWFWMD

K_FLORIDAN 6.87 7.57 -.7 -1 SWFWMD

MPA 4.43 4.85 -.42 -.6 SWFWMD

UNNAMED_1 5.19 5.89 -.7 -.99 SWFWMD

UNNAMED_2 5.37 5.71 -.34 -.48 SWFWMD

WELL#12 6.25 6.12 .13 .18 CTT

WELL#9 5.88 6.15 -.27 -.38 CTT

WOODMONT 6.32 6.25 .0658 .0931 CTT

ON_PARK 5.91 7.71 -1.8 -2.54 SWFWMD

SERENA 5.85 7.18 -1.33 -1.88 CTT

GRADE_ORANGE_SU 5.61 6.53 -.91 -1.28 SWFWMD

Hydraulic head—Surficial aquifer system

BBP-S45 7.47 8.47 -.99 -1.41 TANC

GARC-S23 6.63 7.42 -.79 -1.12 TANC

JARP-S40 6.01 6.96 -.95 -1.34 TANC

Hydraulic head gradient—Upper Floridan aquifer (upper to lower part)

GARC-UL -.4 .11 -.51 -.72 TANC

LRP-UL .0100 .51 -.5 -.7 TANC

MAS-UL -.66 -.640 -.6 -.84 TANC

62SRP-UL 1.071 1.14 -.700 -.989 TANC

RP-UL .0110 .00155 .00945 .0134 TANC

THC-UL .013 .0810 -.680 -.961 TANC

WP-UL .0420 -.56 .61 .86 TANC
32 Simulations of Groundwater Flow and Particle Tracking Analysis ... to a Public-Supply Well near Tampa, FL, 2002-05

Observation 

name

Observed 

value

Simulated 

value Residual Weighted residual Data 

source

Hydraulic head gradient—Surficial aquifer system to the Upper Floridan aquifer

GARC-SF .4 .56 -.17 -.23 TANC

113RC-SF .46 .75 -.28 -.4 TANC

RP-SF .1 .48 -.38 -.53 TANC

TCH-SF .016 .88 -.87 -1.23 TANC

WP-SF 2.15 2.84 -.7 -.98 TANC

LRP-S25 18.1 20 -1.91 -.85 TANC

LP-S30 8.56 8.52 .0330 .0467 TANC

MAS-R-S30 12.65 9.08 3.57 1.59 TANC

113RC-S35 7.31 7.59 -.28 -.39 TANC

QRP-S20 8.64 7.22 1.41 2 TANC

RP-S20 6.67 7.19 -.52 -.73 TANC

THC-SA/H46 1 6.83 7.52 -.68 -.97 TANC

WP-S64 8.38 8.58 -.2 -.28 TANC

62SRP-S34 13.37 12.71 .66 .29 TANC

113RC-H50 2 7.33 7.25 .0767 .11 TANC

1THC-SA penetrates the surficial aquifer system, and H46 penetrates the intermediate confining unit.

2 113RC-H50 penetrates the intermediate confining unit.

Table 6. (Continued) Measured and simulated hydraulic heads and hydraulic head gradients, in the local-scale 

model, and residual, weighted residual, aquifer, and data source for monitoring wells.

[Hydraulic heads, in meters. TANC, Transport of Anthropogenic and Natural Contaminants; SWFWMD, Southwest Florida 

Water Management District; CTT, City of Temple Terrace. PSW is the selected public-supply well] 

Figure 16. Simulated and measured hydraulic heads, hydraulic head gradients, sulfur hexafluoride and tritium concentrations, and 

corresponding weighted/nonweighted simulated and measured values.

WEIGHTED MEASURED VALUES

0

20

15

10

5

0

Upper Floridan aquifer heads, in meters

Upper Floridan aquifer to surficial aquifer system gradients, in meters

Surficial aquifer system to Upper Floridan Aquifer gradients, in meters

SIMULATED HEADS AND NONWEIGHTED

CALCULATED VALUES

SIMULATED HEADS AND WEIGHTED

CALCULATED VALUES

20

15

10

5

0

NONWEIGHTED MEASURED VALUES

0 5 10 15 20 5 10 15 20

EXPLANATION

Surficial aquifer system heads, in meters

Sulfur hexafluoride age tracer concentration, in volumetric parts per trillion

Tritium age tracer concentration, in tritium units
Calibration 33

6.43

8.37

6.62

6.83

6.01

8.56

8.64

7.31

6.73

12.65 12.74

17.26

15.64

18.27

18.66

7.33

TTP-4

(PSW)

7.33

EXPLANATION

Simulated water table

elevation, in meters

above NAVD 88

Depressions

Rivers, streams, and

wetlands

Selected PSW, number is

water level in meters

above NAVD 88

Surficial aquifer system

well, number is water

level in meters above

NAVD 88

Intermediate confining

unit well, number is

water level in meters

above NAVD 88

2.79 to 5.68

>5.68 to 7.07

>7.07 to 8.00

>8.00 to 8.87

>8.87 to 10.11

>10.11 to 11.97

>11.97 to 14.26

>14.26 to 23.24

>23.24 to 45.55

TTP-4

(PSW)

6.43

18.27

0 0.25 0.5 0.75 1 KILOMETERS

0 0.25 0.5 MILES

0 1 2 KILOMETERS

0 1 MILE

TTP-4

(PSW)

Hillsborough

River

Area

shown

above

City of

Temple

Terrace

Local-scale

model

boundary

Figure 17. Simulated and measured hydraulic heads in the surficial 

aquifer system and intermediate confining unit in the middle of the 

local-scale model area. PSW is the selected public-supply well.
34 Simulations of Groundwater Flow and Particle Tracking Analysis ... to a Public-Supply Well near Tampa, FL, 2002-05

7.00

6.00

6.43 6.73

6.24 6.27

7.41

6.68

6.62

6.85

8.10

7.02

6.82

6.81

6.23 6.56

6.57

TTP-4

(PSW)

EXPLANATION

6.43

Simulated potentiometric

surface in the Upper

Floridan aquifer, in

meters above NAVD 88

Depressions

Rivers, streams, and

wetlands

Selected PSW

Observation well, number

is

, number is

water level in meters

above NAVD 88

water level in meters

above NAVD 88

2.79 to 4.29

>4.29 to 5.30

>5.30 to 6.10

>6.10 to 6.58

>6.58 to 6.93

>6.93 to 7.27

>7.27 to 7.61

>7.61 to 8.01

>8.01 to 8.46

>8.46 to 9.14

TTP-4

(PSW)

6.73

0 0.25 0.5 0.75 1 KILOMETERS

0 0.25 0.5 MILES

0 1 2 KILOMETERS

0 1 MILE

TTP-4

(PSW)

Hillsborough

River

Area

shown

above

Local-scale

model

boundary City of

Temple

Terrace

Figure 18. Simulated and measured hydraulic heads in the Upper Floridan 

aquifer in the middle of the local-scale model area. PSW is the selected 

public-supply well.
Calibration 35

28°00’

82°22’30’’

28°00’

82°22’30’’

28°00’

82°22’30’’

28°00’

82°22’30’’

Simulated potentiometric surface of

Upper Floridan aquifer in layer 5,

in meters above NAVD 88

2.79 to 4.29

>4.29 to 5.30

>5.30 to 6.10

>6.10 to 6.58

>6.58 to 6.93

>6.93 to 7.27

>7.27 to 7.61

>7.61 to 8.00

>8.00 to 8.46

>8.46 to 9.14

EXPLANATION

2.79 to 5.65

>5.65 to 6.97

>6.97 to 7.83

>7.83 to 8.62

>8.62 to 9.56

>9.56 to 10.83

>10.83 to 12.43

>12.43 to 17.71

>17.71 to 23.24

>23.24 to 45.00

>45.00 to 85.12

Rivers, streams, and wetlands

Selected public-supply well

Simulated water table elevation,

in meters above NAVD 88

EXPLANATION

TTP-4

(PSW)

Rivers, streams, and wetlands

Selected public-supply well

0 1 2 3 4 KILOMETERS

0 1 2 MILES

A

B

TTP-4

(PSW)

TTP-4

(PSW)

TTP-4

(PSW)

0 1 2 3 4 KILOMETERS

0 1 2 MILES

Figure 19. Simulated potentiometric 

surfaces of the (A) surficial aquifer 

system and (B) Upper Floridan 

aquifer in the local-scale model 

area. PSW is the selected publicsupply well.
36 Simulations of Groundwater Flow and Particle Tracking Analysis ... to a Public-Supply Well near Tampa, FL, 2002-05

Table 7. Simulated water budget with inflow and outflow components to the local-scale 

modeled area.

Budget component

Inflow

(cubic meters 

per day)

Outflow 

(cubic meters 

per day)

Net 

(cubic meters 

per day)

Net 

(cubic meters 

per second)

Drain 0 45,189.46 -45,189.46 -0.52

River leakage 10,612.05 32,054.86 -21,442.81 -.25

Recharge 79,756.98 0 79,756.98 .92

Specified boundary flows 59,574.94 50,875.85 8,699.09 .10

Multi-node wells 4,884.38 26,730.30 -21,845.92 -.25

Total: 154,828.34 154,850.47 -22.13 -.0026

and some of layer 2 were unsaturated, so the corresponding 

cells were made inactive on the terrace. Simulated hydraulic 

heads in layers 1 to 3 ranged from 2.79 to 85.12 m (fig. 19A). 

The highest simulated hydraulic head occurred in only one 

cell in the southeastern and southwestern parts of the model 

area. The second and third highest simulated hydraulic head 

values (45.2 and 40.9 m) occurred in one cell each—next to 

the highest hydraulic head cell and near the selected PSW. 

These cells are areas of focused recharge that likely are bigger 

in size than the model cell or do not drain an area as large as 

the digital elevation model would indicate. The fourth highest 

simulated hydraulic head was 22.8 m.

The simulated potentiometric surface of the Upper 

Floridan aquifer followed the measured potentiometric surface 

of the Upper Floridan aquifer based on median calendar year 

2000 water levels (figs. 18 and 19B). Generally, water flowed 

from the northern and northeastern parts of the local-scale 

model area toward the southern and southeastern parts. The 

highest simulated hydraulic heads occurred on the northwestern part of the model area; on the boundary, outflow 

occurred through the southeastern and southwestern parts of 

the local-scale model area. The ranges in hydraulic heads fit 

observed hydraulic heads reasonably well. The hydraulic head 

decreased from north to south and southeast across the localscale model area, in good agreement with the general observed 

flow direction.

Water Budget

The simulated water budget has two main components 

of inflow (recharge and specified boundary flows) and four 

net outflow components (specified boundary flows, drains, 

river leakage, and multi-node wells) to the model area 

(table 7). The simulated water budget is also in agreement 

with regional studies for recharge, specified boundary flows, 

and river leakage where data are available. Total inflow is 

154,828.34 m3/d, including inflow from recharge, specified boundary flows, river leakage, and multi-node wells. 

Recharge contributes 79,756.98 m3/d into the model and 

accounts for more than 51.5 percent of total inflows (table 7). 

There is a net specified influx across the model boundaries of 

59,574.94 m3/d, which is about 5.6 percent of the total inflow. 

Outflow discharges include flow out of lateral model boundaries, drains, river leakage, and withdrawals by wells. The 

total discharge to drains is 45,189.46 m3/d, or 29.2 percent of 

the total outflow. River leakage is variable by cell within the 

model, but overall there is more outflow than inflow. The net 

river leakage is 21,442.81 m3/d. Most of the leakage from the 

aquifer to the river occurs near the bottom of the reservoir.

Particle Tracking

A summary of final steady-state traveltimes for each 

particle reaching the monitoring wells or the selected PSW 

was obtained from MODPATH and used to describe the 

distribution of simulated particle ages associated with recharge 

water reaching the wells. A tracer concentration was assigned 

to each particle on the basis of the recharge date (from particle 

ages) and the concentration of each age tracer in the atmosphere at the time of recharge (minus radioactive decay in the 

case of tritium concentrations). Atmospheric concentrations of 

sulfur hexafluoride have increased since the 1950s, and sulfur 

hexafluoride concentrations have been used to date recently 

recharged groundwater (Busenberg and Plummer, 2000). In 

addition, concentrations of tritium in precipitation (Michel, 

1989) peaked in the 1960s and have been declining for the 

most part since that time; tritium concentrations were adjusted 

for radioactive decay to 2004 assuming a half-life of 12.3 

years. A flux-weighted average concentration was calculated 

for each well by multiplying the concentration assigned to 

each particle by the amount of recharge associated with that 

particle and summing overall particles reaching a well. The 

calculated tracer concentrations derived from simulated flow 

paths and advective traveltimes were compared with measured 

concentrations and ages.
Calibration 37

0 1 2 3 4 5 6

6

5

4

3

2

1

0

CALCULATED SULFUR HEXAFLUORIDE

CONCENTRATION, IN VOLUMETRIC

PARTS PER TRILLION

MEASURED SULFUR HEXAFLUORIDE CONCENTRATION,

IN VOLUMETRIC PARTS PER TRILLION

One-to-one line

Surficial aquifer system

Intermediate confining

unit

Upper Floridan aquifer

TTP-4 (PSW)

Figure 20. Calculated and measured sulfur hexafluoride 

concentrations in the selected public-supply well (PSW) and 

monitoring wells.

Age Tracer Concentrations

Calculated sulfur hexafluoride concentrations were in good 

agreement with measured values in wells that were screened in 

the surficial aquifer system, intermediate confining unit, and in 

the top of the Upper Floridan aquifer. However, the agreement 

was not as good in wells open deeper in the Upper Floridan 

aquifer (fig. 20; table 8). For example, water from shallow 

monitoring well GARC-S23 (7 m deep) had a measured sulfur 

hexafluoride concentration of 5.27 pptv, whereas the calculated 

concentration was 5.65 pptv [pptv is the volume fraction, in 

parts per trillion, of sulfur hexafluoride in air that would be in 

equilibrium with the water sample in the recharge area, after 

adjustment for excess air (Busenberg and Plummer, 2000)]. 

The measured sulfur hexafluoride concentration was 0.23 

pptv in water from monitoring well THC‐F197 (60 m deep) 

and the calculated concentration was 1.31 pptv. Calculated 

concentrations do not match measured concentrations in deeper 

monitoring wells, probably because of uncertainty in the representation of karst features at that depth and the fact that deeper 

wells have longer flow paths. Also, the model probably does not 

exactly reflect the actual distribution of such features and the 

effects on flow paths. Measured sulfur hexafluoride concentration samples from the selected PSW ranged from 3.81 to 4.26 

pptv, and the calculated value was 3.6 to 3.98 pptv (table 8).

Calculated tritium concentrations in the selected PSW 

from two separate sampling dates were about 2.5 and 2.4 

tritium units (TU) and were slightly higher than the measured 

concentrations of about 2.2 and 2 TU (fig. 21; table 8). Many of 

the calculated concentrations of tritium for the surficial aquifer 

system and intermediate confining unit monitoring wells were 

similar to measured concentrations. For example, the measured 

tritium concentration in well 113RC-S35 was 2.15 TU, and 

the calculated concentration was 2.25 TU. Calculated tritium 

concentrations were generally higher than measured concentrations in Upper Floridan aquifer wells, except for concentrations 

in those wells screened in the conduit layer in the Tampa 

Member, including the selected PSW (fig. 21). The measured 

tritium concentration was 0.04 TU in well GARC‐F200 (open 

to the Upper Floridan aquifer), and the calculated concentration was 2.5 TU. In water from the deeper monitoring wells, 

the poor match between measured and calculated tritium 

concentrations may be a result of model limitations, such as 

inexact karst feature placement, the assumption of steady-state 

transport, and that a large percentage of the recharge water is 

older than tritium-age tracer data can be used for.

Simulated Particle Ages

Simulated particle ages representing the water recharging 

a well and derived from particle tracking cannot be compared 

directly with the apparent age of recharge water derived from 

the concentration of specific atmospheric tracers, such as 

sulfur hexafluoride and tritium, in groundwater. Atmospheric 

tracers yield an apparent age of recharge water for each well 

(Katz and others, 2007), whereas particle tracking yields 

an age distribution of water particles recharging each well. 

However, the apparent age from tracers can be compared to 

the median (or central tendency) of particle ages for each well 

to provide an assurance that the model provides reasonable 

traveltimes for water recharging wells and to give an approximation of land use in the area contributing recharge to each 

well at the time of recharge.

Simulated particle ages for monitoring wells ranged from 

less than 1 day to 99 years (table 9). Overall, particle ages 

agree favorably with apparent ages from atmospheric tracers 

for monitoring wells open to the surficial aquifer system and 

the intermediate confining unit, whereas the mixing of waters 

with a wide range of ages is less likely in wells open to the 

karstic Upper Floridan aquifer. The simulated and probable 

actual median particle ages generally increased with depth and 

hydrogeologic unit. The median simulated particle ages for all 

monitoring wells open to the surficial aquifer system (layers 1 

to 3) was 1.9 years, but the ages ranged from less than 1 day to 

9.5 years, whereas the apparent ages from atmospheric tracers 

ranged from 0 to 9 years (table 9). The median simulated 

particle age for monitoring wells screened in the intermediate 

confining unit ranged from 12.1 to 47.1 years; the maximum 

was 50.0 years (table 9). The apparent age for atmospheric 

tracers ranged from 0 to 14 years. The median ages of simulated particles for all monitoring wells open to the Tampa 

Member (layers 5 to 13, except for layer 8) was 24.3 years, 

but the medians ranged from 1.2 to 96 years (table 9). The 

apparent ages from atmospheric tracers ranged from 22 to 67 

years. The oldest particle ages were simulated for monitoring 

well WP‐F299, which was also the deepest well used in this 

study. The median of simulated particle ages for monitoring 

wells open to the conduit layer (model layer 8) was 14.9 years; 

the median particle ages ranged from 0.3 to 28.1 years. The 

apparent age from atmospheric tracers was 12 years.
38 Simulations of Groundwater Flow and Particle Tracking Analysis ... to a Public-Supply Well near Tampa, FL, 2002-05

Table 8. Measured and simulated sulfur hexafluoride and tritium concentrations in the local-scale model 

and residual, weighted residual, and aquifer name for monitoring wells.

[Data source: Transport of Anthropogenic and Natural Contaminants (TANC). PSW is the selected public-supply well. 

UFA, Upper Floridan aquifer; ICU, intermediate confining unit; SAS, surficial aquifer system]

Short site name Observed 

value

Simulated 

value Residual Weighted 

residual Aquifer

Sulfur hexafluoride concentration, in volumetric parts per trillion

113RC-F190 0.31 0.13 0.17 .0174 UFA

113RC-H50 2.44 2.68 -.24 -.77 ICU

113RC-S35 4.01 4.43 -.42 -1.33 SAS

GARC-F200 1.22 .82 .4 1.26 UFA

GARC-F75 .6 5.33 -4.73 -6.69 UFA

GARC-S23 5.27 5.65 -.38 -1.21 SAS

LP-H40 2.75 2.7 .0461 .15 ICU

LP-S30 2.8 5.53 -2.74 -8.66 SAS

RP-F103 .71 .25 .45 .72 UFA

RP-F77 .91 .27 .64 .91 UFA

RP-S20 4.78 5.52 -.74 -2.33 SAS

THC-F197 .23 1.31 -1.08 -3.42 UFA

THC-F75 .72 4.22 -3.49 -4.94 UFA

THC-SA/H46 4.76 3.06 1.71 5.4 SAS/ICU

WP-F150 2.81 .53 2.28 7.22 UFA

WP-F299 .34 0 .34 1.11 UFA

WP-S64 4.95 5.49 -.54 -1.72 SAS

TTP-4 (PSW) 3.81 3.6 .21 2.1 SAS

PSW AT 49 meters 4.26 3.98 .28 2.78 UFA

Tritium concentration, in tritium units

113RC-F190 0.17 6.09 -5.91 -0.59 UFA

113RC-H50 1.33 1.85 -.52 -.52 ICU

113RC-S35 2.15 2.25 -.1 -.34 SAS

GARC-F200 .0400 2.5 -2.46 -1.74 UFA

GARC-F75 .43 1.87 -1.44 -1.02 UFA

GARC-S23 2.13 2 .13 .4 SAS

LP-H40 2.03 1.61 .42 1.33 ICU

LP-S30 2.58 1.78 .8 .57 SAS

RP-F103 .75 5.57 -4.82 -1.08 UFA

RP-F77 .74 5.37 -4.63 -1.04 UFA

RP-S20 2.22 1.94 .27 .86 SAS

THC-F197 .090 2.29 -2.2 -1.56 UFA

THC-F75 1.11 2.85 -1.74 -1.23 UFA

THC-SA/H46 2.48 2.04 .44 1.41 SAS/ICU

WP-F150 2.41 3.21 -.8 -2.54 UFA

WP-F299 0 .00954 -.095 .00954 UFA

WP-S64 2.59 1.93 .67 2.11 SAS

TTP-4 (PSW) 2.17 2.52 -.35 -3.53 SAS

TTP-4 (PSW) at 49 meters 1.99 2.4 -.41 -4.03 UFA
Comparison of Land Use in the Area Contributing Recharge and Occurrence of Selected Contaminants 39

Simulated particle ages ranged from less than 1 day to 

127 years for the selected PSW, open in the conduit layer of 

the Upper Floridan aquifer, and nearly 45 percent of the 

simulated particle ages for the selected PSW were younger 

than about 10 years (table 9; fig. 22). The median simulated 

particle age for this well was 13.1 years; the average (from 

11 samples) mean apparent age from sulfur hexafluoride 

was 9 years—the tritium age was not available for this well 

(table 9). Additional information on the mean apparent age 

of groundwater derived from age tracer concentrations is 

discussed in Katz and others (2007).

Comparison of Land Use in the Area 

Contributing Recharge and Occurrence 

of Selected Contaminants

The simulated ACR for the selected PSW encompasses 

about 24.3 km2 and extends from 2.5 to 3 km north to 

northwest and 0.5 to 1.0 km south to southeast of the well 

(fig. 22). Although the simulated ACR for the selected PSW 

in Temple Terrace, Florida, likely does not represent the 

“true area contributing recharge,” it is a useful tool for a 

general examination of current and past land uses at the time 

of recharge to approximate contaminants in groundwater 

recharging the PSW. The simulated ACR for the selected 

PSW is represented as a network of model-grid cells 

(some contiguous and others disconnected). The “patchy” 

appearance of the ACR, including the presence of areas of 

short-circuited flow paths and abbreviated traveltimes, is 

One-to-one line

Surficial aquifer system

Intermediate confining unit

Upper Floridan aquifer

TTP-4 (PSW)

MEASURED TRITIUM CONCENTRATION, IN TRITIUM UNITS

0 1 2 3 4 5 6

7

6

5

4

3

2

1

0

CALCULATED TRITIUM CONCENTRATION,

IN TRITIUM UNITS

Figure 21. Calculated and measured tritium concentrations in the 

selected public-supply well (PSW) and monitoring wells.

Table 9. Simulated particle ages, minimum, median, and maximum at 

monitoring wells and at the selected public-suppy well compared to 

the apparent age of the young fraction of water, interpreted from sulfur 

hexafluoride and tritium concentrations by aquifer.

[<, less than; --, no data; ages in years, except where noted; shaded rows are 

wells that represent the conduit layer (layer 8) within the Upper Floridan 

aquifer]

Short site 

name

Particle (simulated) age

Measured apparent 

mean age of young 

fraction of water

Minimum Median Maximum 

Sulfur 

hexafluorideTritium 

Surficial aquifer system

RP-S20 < 4 days 0.5 1.0 3 0

QRP-S20 .2 2.2 5.6 -- --

GARC-S23 < 1 day < 8 days < 11 days 2 9

LRP-S25 .9 .9 1.0 -- --

LP-S30 1.5 1.9 3.6 -- --

62SRP-S34 4.8 6.0 7.3 -- --

113RC-S35 2.0 4.3 9.5 7 4

BBP-S45 1.5 2.9 6.6 -- --

WP-S64 < 33 days .7 1.3 4 5

Intermediate confining unit

LP-H40 13.7 13.9 14.1 14 --

THC-SA-H46 6.6 12.1 12.6 4 0

113RC-H50 12.1 12.3 12.5 14 8

62SRSP-H55 11.8 13.9 16.6 -- --

LRP-H105 44.4 47.1 50.0 -- --

Upper Floridan aquifer

MAS-R-F64 7.6 1.7 13.9 -- --

GARC-F75 1.2 1.2 1.2 27 33

THC-F75 5.5 6.0 6.6 25 29

RP-F77 32.4 32.4 32.4 23 22

RP-F103 33.2 33.2 33.2 25 29

WP-F150 20.5 28.1 42.6 12 12

LRP-F160 16.4 26.5 58.1 -- --

62SRP-F160 3.2 3.3 3.4 -- --

MAS-R-F160 .3 .3 16.2 -- --

TTP-4 (PSW) < 1 day 13.1 127.0 5–10 --

113RC-F190 37.9 43.5 52.5 32 35

THC-F197 16.9 20.3 23.3 36 62

GARC-F200 20.4 24.3 29.1 30 67

WP-F299 91.3 96.3 99.1 31 --
40 Simulations of Groundwater Flow and Particle Tracking Analysis ... to a Public-Supply Well near Tampa, FL, 2002-05

DEPRESSIONS

RIVER, STREAMS,

AND WETLANDS

PATHLINES

TTP-4 (PSW)

TTP-4

(PSW)

0.5

AREA CONTRIBUTING RECHARGE, IN YEARS

EXPLANATION 0 1 2 KILOMETERS

0 0.5 1 MILES

0.5 1.5

0 1 2 KILOMETERS

0 1 MILE

TTP-4

(PSW)

Hillsborough

River

Area

shown

above

City of

Temple

Terrace

Local-scale

model

boundary

< 0 day to 6.4

>6.4 to 16

>16 to 26

>26 to 37

>37 to 50

>50 to 66

>66 to 127

Figure 22. Simulated area contributing recharge for the selected 

public-supply well (PSW) and age of particles derived from simulated 

particle traveltimes.
Long-Term Concentrations of Nitrate in the Public-Supply Well 41

probably a realistic feature of this aquifer system. This finding 

is consistent with the conceptual understanding of the hydrogeology where some flow paths allow rapid flow through 

conduits and others allow slower flow through the relatively 

undisturbed matrix.

Current land use and land cover in the simulated ACR 

for the selected PSW are predominantly urban and residential 

(table 10). Based on recent land-use information (Homer and 

others, 2000), 84 percent of the ACR intersects residential/

commercial land uses, followed by rangeland and wetland 

land covers, with a combined total of 10.6 percent. Only about 

4 percent of the ACR intersects agricultural lands (table 9). 

Within the ACR urban/residential area, about 75 percent of the 

land use is residential (visually apparent on fig. 23) and the 

remaining 9 percent is commercial and barren land.

Simulated ACRs for the monitoring wells are also located 

mainly to the northwest and west, consistent with the conceptualization of the flow system in this area (fig. 24). Land use 

in the ACRs for the monitoring wells in the local-scale model 

area is similar to that of the selected PSW—mainly residential 

with a fraction of commercial and other land uses. Most 

samples from surficial aquifer system monitoring wells had 

nitrate concentrations that ranged from 0.21 to 6.11 mg/L as 

nitrogen. The average concentration of nitrate was 2.4 mg/L 

in monitoring wells screened in the surficial aquifer system. 

Typical VOCs in groundwater from urban land-use settings 

(trichloroethylene, tetrachloroethene, chloroform, and carbon 

disulfide) were found in most samples from surficial aquifer 

system wells. The pesticides atrazine, de-ethyl atrazine, and 

prometon were also found in six samples from the surficial 

aquifer system and intermediate confining unit wells and 

from three retention ponds, as discussed in Katz and others 

(2007). Generally, nitrate concentrations were lower in deeper 

monitoring wells than in the shallower monitoring wells. The 

median nitrate concentration was less than the detection level 

of 0.06 mg/L in most monitoring wells open to the Upper 

Floridan aquifer. Only the selected PSW and two other Upper 

Floridan aquifer wells (MAS-R‐F160 and 62SRP‐F160), 

open to the conduit layer in the Tampa Member (layer 8), had 

nitrate concentrations greater than the detection level.

Long-Term Concentrations of Nitrate in 

the Public-Supply Well

Groundwater is generally oxic in the surficial aquifer 

system, but conditions become anoxic near the interface 

between the surficial aquifer system and the intermediate 

confining unit, and into the Upper Floridan aquifer (Katz 

and others, 2007; McMahon and others, 2008); therefore, 

nitrate is transported conservatively through the surficial 

aquifer system. Near the interface within the intermediate 

confining unit and in the Upper Floridan aquifer, denitrification removes nitrate from groundwater, producing nitrogen 

gas and other byproducts of reactions (McMahon and others, 

2008). Measured nitrate concentrations at the selected PSW 

ranged from 0.6 to 3.6 mg/L, which is consistent with recharge 

to this well from varying mixtures of young water from the 

surficial aquifer system that bypass denitrification processes 

and quickly recharge the well, and older deeper water from 

the Upper Floridan aquifer that is essentially nitrate free. 

Results from geochemical mass-balance mixing models for 

the selected PSW indicate that 50 to 70 percent of water 

withdrawn from this well is contributed from the surficial 

aquifer system, and 30 to 50 percent from the Upper Floridan 

aquifer (Katz and others, 2007). The selected PSW seems to 

be affected by the rapid downward movement (exacerbated by 

pumping) of contaminants from the surficial aquifer system to 

the well by means of sinkholes.

Table 10. Land-use statistics in the 

area contributing recharge area for 

the selected public-supply well, 2000.

Land-use category Percent

Urban 83.73

Agriculture 4.43

Forest .69

Rangeland 6.26

Barren .11

Wetland 4.32

Water .46

Total: 100

 

Mainly urban (industrial and gasoline derived) volatile 

organic compounds (VOCs), such as toluene and trichloroethylene (TCE), and lawn derived compounds, such as atrazine 

and nitrate contaminants, would be expected in the shallow 

groundwater beneath the ACR. The urban area was primarily 

developed in the late 1950s and 1960s with residential and 

commercial development replacing mostly agricultural or 

barren land. Land use in the ACR has remained relatively 

stable since that time. Nitrate and low concentrations of TCE, 

toluene, and atrazine and de-ethyl atrazine (degradation products of the triazine herbicides) are present in the selected PSW 

at levels above the expected background levels, but still below 

any drinking-water standards (Katz and others, 2007). Sources 

of nitrate are most likely from residential/commercial nitrogen 

fertilizer use and atmospheric deposition based on delta 

nitrogen-15/nitrogen-14 (d15N/14N) of nitrate (McMahon and 

others, 2008). The specific VOCs detected in the selected PSW 

are likely derived from urban land-use practices (Zogorski and 

others, 2006). Pesticides detected in the selected PSW could be 

derived from either agricultural or urban land use, but are most 

likely from urban land use in this area (Katz and others, 2007).
42 Simulations of Groundwater Flow and Particle Tracking Analysis ... to a Public-Supply Well near Tampa, FL, 2002-05

TTP-4

(PSW)

EXPLANATION

TTP-4 (PSW)

Underground storage tanks

Retention ponds

Drycleaner

Hazardous waste

Final travel time, in years

0.1 to 6.4

>6.4 to 15.8

>15.8 to 25.7

>25.7 to 36.9

>36.9 to 50.2

>50.2 to 66.4

>66.4 to 127.0

0 1 2 KILOMETERS

0 0.5 1 MILES

0.5 1.5

0 1 2 KILOMETERS

0 1 MILE

TTP-4

(PSW)

Hillsborough

River

Area

shown

above

City of

Temple

Terrace

Local-scale

model

boundary

Figure 23. Simulated area contributing recharge for the 

selected public-supply well (PSW) with simulated traveltimes, 

point sources of contaminants in the local-scale study area, 

and a recent digital-orthographic-quarter-quadrangle depicting 

current land use/land cover.
Long-Term Concentrations of Nitrate in the Public-Supply Well 43

!

! !

0 1 2 KILOMETERS

0 1 MILE

TTP-4

(PSW)

Hillsborough

River

Area

shown

above

City of

Temple

Terrace

Local-scale

model

boundary

TTP-4

0 to 2

>2 to 4

>4 to 9

>9 to 20

>20 to 33

>33 to 58

>58 to 91

DEPRESSIONS

RIVER, STREAMS, AND WETLANDS

PATHLINES

TTP-4 (PSW)

SURFICIAL AQUIFER SYSTEM

PONDS

RIVER STATION

INTERMEDIATE CONFINING UNIT

UPPER FLORIDAN AQUIFER

AREA CONTRIBUTING

RECHARGE AGE, IN YEARS

EXPLANATION 0 1 2 KILOMETERS

0 0.5 1 MILES

0.5 1.5

Figure 24. Simulated area contributing recharge for monitoring wells 

and age of particles derived from simulated particle traveltimes.
44 Simulations of Groundwater Flow and Particle Tracking Analysis ... to a Public-Supply Well near Tampa, FL, 2002-05

Long-term nitrate concentrations in the selected PSW 

will depend primarily on the quantity and timing of recharge 

events, withdrawals, land-use decisions, and concentrations 

of nitrate in the atmosphere over time within the ACR to the 

well. The nitrate concentration input history in local groundwater recharge was estimated by McMahon and others (2008) 

by summing measured nitrate and dissolved excess nitrogen 

gas (N2

) concentrations to obtain initial nitrate concentrations 

at the time of recharge. Denitrification rates were estimated 

using measured and initial nitrogen concentrations along with 

age tracer data. Estimated denitrification rates range from 0.1 

to 0.5 mg/L as N per year in sediments below the surficial 

aquifer system within the local-scale model area.

Nitrate concentrations in the selected PSW were 

calculated over time as part of the present study by using an 

estimated nitrate concentration input history at the water table, 

based on sparse historically reported nitrate concentrations in 

water from wells in the local-scale model area and estimated 

reconstructed input concentrations from McMahon and others 

(2008). When available, a reported or reconstructed input 

value was used. When not available, however, the input was 

varied by using the nitrate concentrations from the surficial 

aquifer system and time of recharge data collected for this 

study. The nitrate input from 1900 to 1975 was estimated to 

be less than 0.84 mg/L as nitrogen (N). During 1975-92, the 

nitrate input was increased to 4.24 mg/L by using a linear 

slope of 0.1 mg/L per 6 months, based on the reconstructed 

input concentration for 1992. During 1992-2002, the estimated 

reconstructed nitrate input concentration varied temporally 

from 1.05 to 4.81 mg/L based on measured nitrate concentrations during that time period. The reconstructed nitrate input 

function for the selected PSW is shown in figure 25 along 

with measured values and estimated concentrations based 

on a range of denitrification rates. Variability in nitrate input 

concentrations are probably a more realistic way to simulate 

the input function because fertilizers are usually applied 

episodically, and whether or not nitrate reaches the water table 

also depends on the timing of recharge events, plant uptake, 

and other factors. Denitrification rates were also varied for 

each hydrogeologic unit based on given maximum, minimum, 

and mean values. Calculated nitrate concentrations in the 

selected PSW were estimated by using the same approach 

(flux weighted) as was used with sulfur hexafluoride and 

tritium comparisons with measured concentrations.

Input nitrate concentrations were flow weighted and 

tracked through model layers representing the different 

oxidation-reduction zones in the surficial aquifer system, 

intermediate confining unit, and Upper Floridan aquifer. 

Nitrate inputs were not modified, because particles were 

tracked through model layers 1 to 3, representing the oxic 

surficial aquifer system, where little or no denitrification is 

thought to occur (McMahon and others, 2008). As particles 

are tracked downward, reducing conditions begin near the 

interface within the intermediate confining unit and continue 

into the Upper Floridan aquifer (model layers 4 and greater). 

Median (0.3 per year), minimum, and maximum rates, applied 

to the flow-weighted nitrate input concentrations, were 

assumed for particles as they were tracked from the water 

table to the selected PSW. The final calculated nitrate concentration at the selected PSW over time is given as the sum of 

flow-weighted nitrate concentrations of all particles reaching 

the selected PSW for each year.

The final nitrate concentration calculated for the selected 

PSW agreed relatively well with measured values and varied 

with the input function over time (figs. 25 and 26). Because 

the input history is somewhat uncertain over time, this study 

focuses mainly on the distribution of calculated concentrations from 1992 to 2006, where there are more reconstructed 

nitrate input data for the recharging waters. Also, land use 

was relatively stable over this period. Final calculated nitrate 

concentrations in the selected PSW varied from 0.80 to 

2.05 mg/L with a median and mean concentration of 1.33 and 

Simulated nitrate concentration in the selected PSW using

0.5 milligrams per liter as N per year denitrification rate

Simulated nitrate concentration in the selected PSW using

0.3

Modified reconstructed INPUT nitrate concentration function

Simulated nitrate concentration in the selected PSW using

0.5 m

OBSERVED NITRATE CONCENTRATION

Mean in selected PSW for 2003 and 2004

Minimum in selected PSW in 2003 and 2004

Maximum in selected PSW in 2003 and 2004

Mean in surficial aquifer system wells in 2003 and 2004

Minimum in surficial aquifer system wells in 2003 and 2004

Maximum in surficial aquifer system wells in 2003 and 2004

Reconstructed nitrate concentrations, from McMahon and others

(2008)

milligrams per liter as N per year denitrification rate

illigrams per liter as N per year denitrification rate

YEAR

1950 1960 1970 1980 1990 2000 2010

7

6

5

4

3

2

1

0

NITRATE CONCENTRATION AS N,

IN MILLIGRAMS PER LITER

EXPLANATION

Figure 25. Estimated nitrate input concentrations, 

reconstructed nitrate in recharge concentrations, estimated 

nitrate concentrations at the selected public-supply well 

(PSW) from flow-weighted particle tracking using median, 

minimum, and maximum denitrification rates, and measured 

nitrate concentrations at the selected PSW and surficial 

aquifer system wells sampled in 2003 and 2004.
Summary and Conclusions 45

1.36 mg/L, respectively. The median nitrate concentration 

from water samples collected in 2003 and 2004 from the 

selected PSW was 1.44 mg/L, and the average concentration 

was 1.54 mg/L—slightly higher than the calculated concentrations (fig. 26). The range in nitrate concentrations (0.61 to 

3.6 mg/L) from water samples from the selected PSW was 

greater than the range of calculated values, but the central 

tendencies are represented relatively well. This may indicate 

that the input function does not vary enough in time and (or) 

space to adequately represent the true variability of nitrate 

concentrations in recharge water entering the groundwater 

flow system. Also, calculated concentrations likely will not 

match measured concentrations exactly, because the groundwater flow model represents steady-state conditions and 

does not simulate individual recharge or fertilizer application 

episodes. For comparison, compliance concentrations of 

nitrate reported for the blended treated water samples from 

Temple Terrace selected PSWs from 1999 to 2001 ranged 

from 1.2 to 2.4 mg/L (based on calculated values and four 

samples). Assuming that recent (1990s and later) land-use 

activities remain relatively constant over time, nitrate concentrations in the selected PSW will likely remain relatively 

constant for the foreseeable future and will remain well 

below the maximum contaminant level for nitrate of 10 mg/L 

as nitrogen.

The vulnerability of the selected PSW to nonpoint 

source inputs of nitrate, pesticides, and (or) other contaminants, such as VOCs, is evident from the water chemistry 

data (Katz and others, 2007) and the nitrate calculations 

presented in this report. Assuming that present groundwater 

flow conditions persist into the future, long-term concentrations of nitrate and VOCs and pesticides in the selected PSW 

will depend on the input of contaminants at the water table 

and recharge events in the future.

The calculated long-term nitrate concentrations in the 

selected PSW also reflect a difference of 1 to 10 years between 

peak input concentrations and peak concentrations in the well. 

This apparent timelag between input concentrations over time 

and the resulting concentrations in the well are the result of 

the wide range of ages of water reaching the selected PSW, 

the variability in input over time, and denitrification rates. An 

understanding of the length of time between changes in input 

concentrations and resulting concentrations in the selected 

PSW is needed to predict the impact of changes in land-use 

management.

To the extent that the conditions affecting the vulnerability of the selected PSW are typical of conditions affecting 

other selected PSWs in the region, these results can be useful 

in evaluating the vulnerability of other wells to nonpoint 

source input of contaminants. Patchy high nitrate concentrations have been observed in shallow groundwater beneath 

urban land throughout the regional-scale study area. About 

75 percent of the public-supply wells in the regional study 

area with open intervals beginning at depths similar to the 

selected PSW may be similarly impacted by nitrate, pesticides, 

and VOC concentrations. Public-supply wells with high 

pumping rates, open intervals at shallow depths, and in close 

proximity to karst features may receive appreciable flows of 

young water that would likely have higher concentrations of 

nitrate, pesticides, and VOCs, because these wells will likely 

have less mixing with old water containing low contaminant 

concentrations.

Summary and Conclusions

In 2002, a single public-supply well, TTP-4, was selected 

for intensive study to evaluate the dominant processes 

affecting the vulnerability of public-supply wells in the Tampa 

Bay region, Florida. A network of 29 monitoring wells was 

installed to support a detailed analysis of physical and chemical conditions and processes affecting the water chemistry of 

the selected public-supply well (PSW). A three-dimensional, 

steady-state groundwater flow model was developed for this 

study to evaluate the age of groundwater reaching the well and 

to test hypotheses on the vulnerability of the selected PSW 

to nonpoint sources of contaminants. The local-scale model 

is nested within the steady-state regional model and was 

calibrated for calendar year 2000. The local-scale model was 

discretized into a uniform grid of finite-difference cells with 

80 rows and 69 columns of 125 meters length on a side. The 

local-scale model grid has 13 layers of variable thickness. The 

active area of the model coincides with the water table at the 

top of the model and the bottom of the regional-scale model 

layer 13, which is below the bottom of the PSW and below the 

dominant pumping zone in the local-scale model area.

NITRATE CONCENTRATION ,

IN MILLIGRAMS PER LITER

AS N

TTP-4 (PSW),

BASED ON 11 SAMPLES

2003-2004

CALCULATED

CONCENTRATIONS

1992-2006

4

3

2

1

0

Figure 26. Nitrate concentrations in the selected public-supply 

well (PSW) sampled in 2003 and 2004 compared to calculated 

concentrations from 1992 to 2006.
46 Simulations of Groundwater Flow and Particle Tracking Analysis ... to a Public-Supply Well near Tampa, FL, 2002-05

The regional-scale model was used to specify stresses 

in the local-scale model, including specified flow on model 

boundaries and groundwater withdrawals. Recharge rates in 

the local-scale model ranged from a base value of 25.6 up to 

1,610.2 centimeters per year (in one cell) of the model grid. 

Recharge was increased where karst features (closed-basin 

depressions) were known or thought to be present. Lateral 

boundaries in the local-scale model were derived from 

the regional-scale model with specified fluxes assigned to 

boundary cells. Withdrawals from public-supply wells were 

determined from reported permitted average withdrawal rates 

in the year 2000—relatively stable for this area. Withdrawal 

rates for agricultural and industrial wells were based on 

previous studies.

The flow system is composed of the surficial aquifer 

system, intermediate confining unit, and freshwater part of the 

Upper Floridan aquifer representing sand, clay, and limestone 

and dolostone, respectively. The model was calibrated using 

systematic manual calibration techniques, MODFLOW-2000, 

and finally UCODE parameter estimation techniques. The 

hydraulic and other parameters that were used, including 

recharge, anisotropy, and porosity, were modified until a best 

fit solution to hydraulic heads, hydraulic gradients, and age 

tracer concentrations was achieved for groundwater flow and 

subsequent particle tracking simulations and tracer concentration calculations.

Particle tracking was used to compute flow paths and 

advective traveltimes in the model area. The computed area 

contributing recharge (ACR) for the selected PSW covered 

about 24.3 square kilometers, extending from about 0.5 to 1.0 

kilometers to the south and southeast of the well to about 2.5 

to 3 kilometers to the north and northwest of the well, underlying a predominantly urban residential land-use area. The 

simulated ages of particles reaching the selected PSW ranged 

from 1 day to 127.0 years with a median of 13.1 years.

Groundwater older than about 80 years within the ACR 

would be expected to contain primarily agricultural or related 

contaminants, whereas groundwater younger than 80 years 

would be expected to contain both urban and agricultural 

contaminants since urban development began to fill in the 

local-scale model area in the 1950s. Elevated concentrations of 

nitrate and low concentrations of volatile organic compounds 

and pesticides were observed in water from the selected PSW, 

which is consistent with mainly urban sources.

Nitrate concentrations in the ACR to the selected PSW 

are highest in shallow groundwater. The median nitrate 

concentration in wells open to the surficial aquifer system 

beneath the urban area was 2.4 mg/L (milligrams per liter). 

Groundwater is generally oxic in the surficial aquifer system; 

therefore, nitrate is expected to be transported conservatively 

through that part of the system. Nitrate concentrations 

decreased with depth to background concentrations of less 

than 0.06 mg/L in monitoring wells screened in the Upper 

Floridan aquifer due to denitrification in the intermediate 

confining unit and Upper Floridan aquifer. Denitrification 

rates varied between 0.1 and 0.5 mg/L as N per year.

The nitrate concentration in the selected PSW (median 

1.44 mg/L) is intermediate between the shallow and deep 

parts of the zone of contribution because of mixing of relatively young and older groundwater at the well. Because of 

the rapid and dominant downward movement of water in 

the local-scale model area through sinkholes exacerbated 

by pumping, nitrate in shallow groundwater in the surficial 

aquifer system is expected to continue to reach the selected 

PSW with time. Estimated nitrate concentrations in recharge 

over time, represented by using observed nitrate concentrations and corresponding groundwater recharge dates, indicate 

overall relatively steady concentrations of nitrate in recharge 

over time in the area contributing recharge. Assuming that 

the proportion of urban land use remains fixed at the current 

proportion of 84 percent urban land use and input concentrations beneath these settings remain relatively constant, 

calculated long-term nitrate concentrations in the selected 

PSW indicate that concentrations will continue to be about 1 

to 3 mg/L for the foreseeable future. The calculated long-term 

nitrate concentrations in the selected PSW also indicate a 

1- to 10-year timelag between changes in input concentrations 

in groundwater recharge and resulting concentrations in the 

well. An understanding of the length of time between changes 

in input concentrations and resulting concentrations in the 

selected PSW is needed to predict the impact of changes in 

land use.

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settings and ground-water flow simulations for regional studies 

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 49

Appendix

The regional ground-water flow model, documented in Crandall 

(2007), was updated and optimized using parameter estimation in 

MODFLOW 2000 to improve recharge estimates in the local-scale 

model area, because this model was used to develop boundary 

conditions along the local-scale TANC model boundary. Hydraulic 

conductivity and recharge parameters were optimized by lumping 

similar zones to reduce the number of parameters and to eliminate 

correlation problems. The following figures present the final updated 

values for horizontal hydraulic conductivity for the Upper Floridan 

aquifer (fig. A-1) vertical hydraulic conductivity of the Upper Floridan 

aquifer (fig. A-2); the updated recharge optimized values (fig. A-3); 

and the horizontal hydraulic conductivity for the surficial aquifer 

system (fig. A-4).
50 Simulations of Groundwater Flow and Particle Tracking Analysis ... to a Public-Supply Well near Tampa, FL, 2002-05

EXPLANATION

0.83

2.3

428.0

931.6

1,083.5

12,614.2

1

0 5 10 KILOMETERS

Calibrated horizontal

hydraulic conductivity of

the Upper Floridan aquifer,

in meters per day

82°45’ 82°30’ 82°15’ 82°00’

28°30’ 28°15’ 28°00’

Figure A-1. Updated estimates of horizontal hydraulic conductivity of the Upper Floridan aquifer for the regional model that was 

used to estimate flow at the boundary of the local-scale model.
Appendix 51

EXPLANATION

All other values

.000031

.0031

.031

.11

0 5 10 KILOMETERS

Calibrated vertical

hydraulic conductivity of

the Upper Floridan aquifer,

in meters per day

82°45’ 82°30’ 82°15’ 82°00’

28°30’ 28°15’ 28°00’

Figure A-2. Updated estimates of vertical hydraulic conductivity of the Upper Floridan aquifer for the regional model that was 

used to estimate flow at the boundary of the local-scale model.
52 Simulations of Groundwater Flow and Particle Tracking Analysis ... to a Public-Supply Well near Tampa, FL, 2002-05

EXPLANATION

3.7

18.6

10.4

38.6

53.0

47.6

3.7

0 5 10 KILOMETERS

Calibrated recharge, in

centimeters per year

82°45’ 82°30’ 82°15’ 82°00’

28°30’ 28°15’ 28°00’

Figure A-3. Updated estimates of recharge for the regional model that was used to estimate flow at the boundary of the 

local-scale model.
Appendix 53

0 5 10 KILOMETERS

EXPLANATION

All other values

12.4

18.1

119.5

1

Calibrated horizontal

hydraulic conductivity of

the surficial aquifer, in

meters per day

82°45’ 82°30’ 82°15’ 82°00’

28°30’ 28°15’ 28°00’

Figure A-4. Updated estimates of horizontal hydraulic conductivity for the surficial aquifer (layer 1) in the regional model that 

was used to estimate flow at the boundary of the local-scale model.