WRIR 02-4033


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Lee, T.M., 2002, Factors Affecting Ground-Water Exchange and Catchment Size for Florida Lakes in Mantled Karst Terrain: Water-Resources Investigations Report 02-4033, 53 p.

ABSTRACT:

In the mantled karst terrain of Florida, the size of the catchment delivering ground-water inflow to lakes is often considerably smaller than the topographically defined drainage basin. The size is determined by a balance of factors that act individually to enhance or diminish the hydraulic connection between the lake and the adjacent surficial aquifer, as well as the hydraulic connection between the surficial aquifer and the deeper limestone aquifer. Factors affecting ground-water exchange and the size of the ground-water catchment for lakes in mantled karst terrain were examined by: (1) reviewing the physical and hydrogeological characteristics of 14 Florida lake basins with available ground-water inflow estimates, and (2) simulating ground-water flow in hypothetical lake basins. Variably-saturated flow modeling was used to simulate a range of physical and hydrogeologic factors observed at the 14 lake basins. These factors included: recharge rate to the surficial aquifer, thickness of the unsaturated zone, size of the topographically defined basin, depth of the lake, thickness of the surficial aquifer, hydraulic conductivity of the geologic units, the location and size of karst subsidence features beneath and onshore of the lake, and the head in the Upper Floridan aquifer.

Catchment size and the magnitude of ground-water inflow increased with increases in recharge rate to the surficial aquifer, the size of the topographically defined basin, hydraulic conductivity in the surficial aquifer, the degree of confinement of the deeper Upper Floridan aquifer, and the head in the Upper Floridan aquifer. The catchment size and magnitude of ground-water inflow increased with decreases in the number and size of karst subsidence features in the basin, and the thickness of the unsaturated zone near the lake. Model results, although qualitative, provided insights into: (1) the types of lake basins in mantled karst terrain that have the potential to generate small and large amounts of ground-water inflow, and (2) the location of ground-water catchments that could be managed to safeguard lake water quality. Knowledge of how ground-water catchments are related to lakes could be used by water-resource managers to recommend setback distances for septic tank drain fields, agricultural land uses, and other land-use practices that contribute nutrients and major ions to lakes.

CONTENTS:

Abstract
Introduction
Purpose and Scope
Acknowledgments
Background
Physical Characterization of Lake Basins
Methods
Physical Characteristics
Topography and Ground-Water Flow Patterns
Hydrogeologic Framework
Numerical Modeling of Ground-Water Flow
Methods
Radial Models
Model Boundaries
Limits to Hypothetical Steady-State Simulations
Limits to Hypothetical Transient Simulations
Steady-State Simulation Results
Initial Lake Basin Model
Effect of Recharge Rate
Effect of Basin Size
Effect of Surficial Aquifer Conductivity
Effect of Intermediate Confining Unit
Effect of Lake Sediment
Effect of Upper Floridan Aquifer Boundary Condition
Effect of Lake Stage
Effect of Lake Depth
Transient Simulation Results
Factors Affecting Ground-Water Exchange and Catchment Size
Catchment Size in Mantled Karst Terrain
Hydrogeologic Controls on Ground-Water Exchange
Summary and Conclusions
References

FIGURES:

1. Schematic showing generalized hydrogeologic section through a Florida ridge lake in a flow-through setting
2-14. Maps showing:
2. Locations of the study lakes in Florida
3. Topographic setting and general direction of ground-water flow in the surficial aquifer around Lake Annie
4. Topographic setting of Lake Barco, general direction of ground-water flow in the surrounding surficial aquifer, and the simulated steady-state ground-water catchment
5. Topographic setting of Lake Five-O, general direction of ground-water flow in the surrounding surficial aquifer, and the simulated steady-state ground-water catchment
6. Topographic setting and general direction of ground-water flow in the surficial aquifer around Lake George and Grassy Lake
7. Topographic setting and general direction of ground-water flow in the surficial aquifer around Halfmoon Lake
8. Topographic setting and general direction of ground-water flow in the surficial aquifer around Lake Hollingsworth
9. Topographic setting and general direction of ground-water flow in the surficial aquifer around Lake Isis
10. Topographic setting and general direction of ground-water flow in the surficial aquifer around Lake Lucerne
11. Topographic setting and general direction of ground-water flow in the surficial aquifer around Lake Olivia
12. Topographic setting and general direction of ground-water flow in the surficial aquifer around Round Lake and Saddle Blanket Lakes
13. Topographic setting of Lake Starr, general direction of ground-water flow in the surrounding surficial aquifer, and the simulated steady-state ground-water catchment
14. Topographic setting and general direction of ground-water flow in the surficial aquifer around Swim Lake
15-20. Cross sections showing simplified hydrogeologic sections along hillsides in the topographic basins of:
15. Lake Annie and Lake Barco
16. Lake Five-O, Lake George, Grassy Lake, and Halfmoon Lake
17. Lake Hollingsworth and Lake Isis
18. Lake Lucerne and Lake Olivia
20. Lake Starr and Swim Lake
21. Graph showing relation of the head difference between the lake and Upper Floridan aquifer to mantle thickness near lake and to lake elevation
22-24. Schematics of:
22. The initial lake basin model
23. The hillsides used for transient simulations
24. Simulation results for the initial lake basin model: areal view of basin showing the location of the ground-water catchment to the lake, and radial view showing the simulated water table and location of the ground-water catchment.
25. Graph showing ground-water flow distribution along the lakebed for the initial lake basin model
26-29. Bar charts showing simulated ground-water inflow and leakage rates, and catchment sizes for modeled lakes with different:
26. Recharge rates
27. Size basins
28. Horizontal hydraulic conductivity (Kh) values in the surficial aquifer. 37
29. Vertical hydraulic conductivity (Kv) values in the intermediate confining unit surrounding the lake
30. Graph showing ground-water flow distribution along the lakebed for different vertical hydraulic conductivity (Kv) values in the intermediate confining unit surrounding the lake
31. Schematic showing different size collapse features beneath lake and various arrangements of lake sediment used in ground-water flow simulations
32-36. Bar charts showing:
32. Simulated ground-water inflow and leakage rates for modeled lakes with different size collapse features beneath the lake
33. Simulated ground-water inflow and leakage rates and catchment sizes for modeled lakes with breaches in the intermediate confining unit surrounding the lake
34. Simulated ground-water inflow and leakage rates for modeled lakes with different arrangements of lake sediment
35. Simulated ground-water inflow and leakage rates and catchment sizes for modeled lakes with different head values in the Upper Floridan aquifer
36. Simulated ground-water inflow and leakage rates and catchment sizes for modeled lakes maintained at different lake stages
37. Schematic showing different lake depths and surficial aquifer thicknesses used in model simulations
38. Bar chart showing simulated ground-water inflow and leakage rates, and catchment sizes for modeled lakes of different depths with the surficial aquifer thickness near lake equal to 41 feet and 57.4 feet
39-40. Graphs showing:
39. Transient ground-water inflow to modeled lakes for basins with different unsaturated zone thicknesses
40. Relation between lake size, the distance offshore that ground-water inflow occurs, and the percentage of the lakebed receiving ground-water inflow

TABLES:

1. Relation of stratigraphic and hydrogeologic units in central Florida
2. Physical characteristics of selected lake basins in Florida
3. Hydrogeologic characteristics of selected lake basins in Florida
4. Hydraulic parameters used in the initial lake basin model
5. Simulated water-table slopes and catchment sizes resulting from
varying horizontal hydraulic conductivity (Kh) in the surficial aquifer
6. Simulated ground-water inflow to lakes adjacent to level and steep hillsides