WATER RESOURCES AND EFFECTS OF GROUND-WATER DEVELOPMENT IN PASCO COUNTY, FLORIDA By J.D. Fretwell U.S. GEOLOGICAL SURVEY Water-Resources Investigations Report 87-4188 Prepared in cooperation with PASCO COUNTY, FLORIDA Tall-ahassee, Florida 1988 DEPARTMENT OF THE INTERIOR DONALD PAUL HODEL, Secretary U.S. GEOLOGICAL SURVEY Dallas L. Peck, Director For additional information Copies of this report may be write to: purchased from: District Chief U.S. Geological Survey U.S. Geological Survey Books and Open-File Reports Section Suite 3015 Federal Center, Bldg. 810 227 North Bronough Street Box 25425 Tallahassee, Florida 32301 Denver, Colorado 80225 CONTENTS Page Abstract ----------------------------------------------------------------- 1 Introduction ------------------------------------------------------------- 2 Purpose and scope ---------------------------------------------------- 4 Previous studies ----------------------------------------------------- 4 Methods of investigation --------------------------------------------- 5 Data-collection sites ------------------------------------------------ 6 Acknowledgments ------------------------------------------------------ 6 Factors affecting the water resources ------------------------------------ 11 Geography, topography, and drainage ---------------------------------- 11 Climate -------------------------------------------------------------- 11 Land use ------------------------------------------------------------- 14 Water use ------------------------------------------------------------ 14 Permitted pumping rates ----------------------------------------- 24 Well fields ----------------------------------------------------- 24 Projected ground-water withdrawals ------------------------------ 27 Hydrogeologic framework ---------------------------------------------- 29 Solution cavities and sinkholes -------------------------------------- 32 Ground-water resources --------------------------------------------------- 35 Surficial aquifer ---------------------------------------------------- 37 Floridan aquifer system ---------------------------------------------- 39 Potentiometric surface ------------------------------------------ 43 Recharge and discharge ------------------------------------------ 45 Aquifer properties ---------------------------------------------- 47 Surface-water resources -------------------------------------------------- 47 Streams -------------------------------------------------------------- 47 Lakes ---------------------------------------------------------------- 52 Springs -------------------------------------------------------------- 56 Quality of water --------------------------------------------------------- 57 Effects of ground-water development on water resources ------------------- 67 Ground-water flow model ---------------------------------------------- 67 Conceptual model and model input -------------------------------- 71 Calibration ----------------------------------------------------- 76 Validation ------------------------------------------------------ 80 Sensitivity analysis -------------------------------------------- 80 Potential effects of future development ------------------------------ 86 Limitations of model application ------------------------------------- 117 Summary and conclusions -------------------------------------------------- 118 Selected references ------------------------------------------------------ 121 Appendixes --------------------------------------------------------------- 125 A. Wells from which ground-water data were collected ---------------- 126 B. Chemical analyses of water from wells ---------------------------- 140 C. Concentrations of chloride, specific conductance, and temperature for selected wells --------------------------------- 166 D. Data-collection sites on streams, lakes, springs, and sinkholes ------------------------------------------------------ 182 E. Chemical analyses of water, stage, and discharge from streams, lakes, springs, and sinkholes ---------------------------------- 187 F. Water levels in the surficial aquifer, September 1984 ------------ 206 ILLUSTRATIONS Page Figures 1-7. Maps showing: 1. Pasco County--the study area -------------------------- 3 2. Location of wells where ground-water data were collected in western (A) Pasco County --------------- 7 3. Location of wells where ground-water data were collected in central (B) Pasco County --------------- 8 4. Location of wells where ground-water data were collected in eastern Pasco (C) County --------------- 9 5. Data-collection sites on streams, lakes, springs, and sinkholes ------------------------------------------- 10 6. General topography of Pasco County -------------------- 12 7. Major drainage basins, tributary divides, and streams - 13 8. Graph showing normal monthly and 1984 monthly rainfall at St. Leo and Tarpon Springs ------------------------------- 15 9. Graph showing annual rainfall at St. Leo, 1931-85 ---------- 16 10. Map showing estimated land use in 1985 --------------------- 17 11. Map showing major population centers and estimated populations for 1980 ------------------------------------- 18 12-16. Graphs showing: 12. Past and projected population of Pasco County --------- 19 13. Estimated freshwater use in 1984 ---------------------- 21 14. Irrigation water use in 1984 -------------------------- 21 15. Monthly irrigation in 1984 ---------------------------- 22 16. Freshwater withdrawal by use category, 1970, 1975, and 1977-84 ----------------------------------------- 23 17. Map showing pumping centers and permitted average daily withdrawals of ground water, 1983 ------------------------ 25 18. Map showing well-field areas in Pasco, Pinellas, and Hillsborough Counties ------------------------------------ 26 19. Generalized geologic sections ------------------------------ 31 20-24. Maps showing: 20. Generalized thickness of the upper confining unit of the Upper Floridan aquifer ----------------------- 33 21. Generalized thickness of surficial sand unit ---------- 34 22. Zones of different sinkhole types and larger sinkholes known or suspected to have connections to the Upper Floridan aquifer ------------------------------------ 36 23. Locations from which sediment samples were collected and estimated elevation of the water table in Pasco County in September 1984 ---------------------- 38 24. Potentiometric surface of the Upper Floridan aquifer in the vicinity of Pasco County showing ground- water flow paths, May 1984 -------------------------- 40 25. Hydrographs showing water levels in three pairs of wells in the surficial aquifer and Upper Floridan aquifer ------ 41 26. Hydrographs showing water levels in wells 285 and 286 ------ 42 27-29. Maps showing: 27. Thickness of the Upper Floridan aquifer --------------- 44 28. Recharge and discharge areas of the Upper Floridan aquifer in Pasco County ----------------------------- 46 29. Aquifer-test sites where transmissivity of the Upper Floridan aquifer was derived ------------------------ 43 IV ILLUSTRATIONS--Continued Page Figure 30. Graphs showing flow-duration curves for major rivers in Pasco County ----------------------------------------------- 50 31. Hydrographs showing water levels in eight lakes in Pasco County ----------------------------------------------------- 53 32. Hydrographs showing water levels in Curve Lake, Pasco Lake, and Black Lake --------------------------------------------- 54 33. Hydrographs showing water levels in Clear Lake, Lake lola, and Crews Lake (North) ------------------------------------- 55 34. Map showing sites where concentrations of dissolved iron were greater than or equal to 300 micrograms per liter in water from the Upper Floridan aquifer ---------------------------- 60 35. Map showing specific conductance of water in the Upper Floridan aquifer ------------------------------------------- 62 36. Stiff diagrams showing concentrations of major constituents in water from selected wells ------------------------------- 63 37. Graph showing concentrations of chloride in water from wells 11 and 31 in the coastal area, 1971-85 --------------- 64 38. Graph showing concentrations of chloride in water from wells 231, 340, and 423 in the coastal area, 1971-85 ------- 65 39. Graph showing concentrations of chloride in water from wells 32, 180, and 336, 1971-85 ---------------------------- 66 40. Cross sections of the saltwater-freshwater transition zone in the Upper Floridan aquifer ------------------------------ 68 41. Map showing model grid and physiographic provinces ----------- 69 42. Diagram showing generalized conceptual model of the hydrogeologic system --------------------------------------- 74 43-45. Maps showing: 43. Comparison of average-observed potentiometric surface and model-calculated potentiometric surface, 1976-77, representing calibration ------------------------------ 77 44. Comparison of average-estimated water table and model- calculated water table, 1976-77, representing calibration ------------------------------------------- 78 45. Comparison of predevelopment potentiometric surface and model-simulated predevelopment potentiometric surface representing model validation ------------------------- 81 46-47. Graphs showing: 46. Effects along row 24 of varying evapotranspiration and recharge parameters on the predevelopment model ------- 83 47. Effects along row 24 of varying aquifer and confining bed hydraulic properties in the predevelopment model -- 84 48-57. Maps showing estimated drawdown in the potentiometric surface under: 48.