Introduction 2 Purpose and scope 4 Area of study and physiography 5 Ground-water use 8 Methods of investigation 8 Previous studies 9 Well-and surface-water-station numbering system 10 Acknowledgments 11 Hydrogeology 11 Geologic setting 11 Hydrologic setting 14 Hydrologic characteristics 14 Overlying semiconfining units 14 Intermediate system 15 Underlying semiconfining Unit 15 Upper Floridan aquifer 17 Lower confining unit 17 Ground water levels 17 Seasonal fluctuations 19 Long term effects of drought conditions and pumpage 20 Effects of surface water features 20 Ground-water quality 22 Surface water 22 Drainage 22 Streamflow 23 Dams and navigational improvements 24 Evaluation of ground-water resources 27 Conceptualization of the flow system 28 Mathematical model 32 Governing equation 32 Boundary and initial conditions 34 Numerical model 34 Simulation approach 34 Steady-state analysis 36 Limitations 37 Advantages 38 Transient analysis 39 Finite element mesh 40 Boundary conditions 41 Regional ground water flow 41 Flow across streambeds 41 Vertical leakage 43 Springflow 45 Hydraulic property zones 46 Distribution of ground-water withdrawal 47 Calibration to October 1986 conditions 49 Ground-water-level residuals 49 iv

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View looking east along the fixed-crest spillway during construction of Jim Woodruff Lock and Dam. During 2002, the U.S. Army Corps of Engineers and the U.S. Geological Survey participated in a dye-tracing study that found lake water moving through the groundwater system to the Apalachicola River via Polk Lake Spring.