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Scientific Investigations Report 2008–5231

Prepared in cooperation with the
National Water-Quality Assessment Program Transport of Anthropogenic and
Natural Contaminants (TANC) to Public-Supply Wells

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

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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 public-supply 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.

Posted November 19, 2009

For additional information contact:
Christy Crandall
crandall@usgs.gov
U.S. Geological Survey
Florida Integrated Science Center
Suite A200
2639 North Monroe St.
Tallahassee, FL 32303
850-553-3674

http://fl.water.usgs.gov/

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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.



Contents

Foreword

Acknowledgments

Abstract

Introduction

Purpose and Scope

Previous Studies

Description of Study Area

Climate

Population and Land Use

Physiography

Surface-Water and Groundwater Interactions

Hydrogeologic Setting

Geology

Lithology

Groundwater Flow System

Methods

Design of Sampling Network and Data Collection and Analysis

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

Groundwater Flow and Advective Tracking Simulation

Model Uncertainties and Limitations

Model Geometry and Discretization

Boundary Conditions and Model Stresses

Description of Lateral Boundaries

Simulation of Recharge and Discharge

Pumping

Hydraulic and Other Estimated Parameters

Hydraulic Parameters

Recharge Parameters

Other Parameters

Parameter Sensitivity

Simulation of Particle Tracking

Calibration

Water Levels and Water-Level Gradients

Flow Patterns

Water Budget

Particle Tracking

Age Tracer Concentrations

Simulated Particle Ages

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

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

Summary and Conclusions

References Cited

Appendix



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