Scientific Investigations Report 2006-5320
Prepared in cooperation with
Polk County Board of County Commissioners
South Florida Water Management District
Southwest Florida Water Management District
St. Johns River Water Management District
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Purpose and Scope
Description of Study Area
Population, Industry, and Land Use
Chapter 1—Ground-Water Resources of Polk County
Surficial Aquifer System
Intermediate Confining Unit and Intermediate Aquifer System
Floridan Aquifer System
Upper Floridan Aquifer
Middle Semiconfining Unit and Middle Confining Unit
Lower Floridan Aquifer
Sub-Floridan Confining Unit
Ground-Water Flow System
Recharge and Discharge
Long-Term Trends in Ground-Water Levels
Surficial Aquifer System
Intermediate Aquifer System
Floridan Aquifer System
Vertical Distribution of Chloride and Sulfate Concentrations
Chapter 2—Streamflow and Lake-Level Characteristics in Polk County
Purpose and Scope
Streamflow and Lake-Level Characteristics
Peace River Basin
Kissimmee River Basin
Alafia, Hillsborough, Ocklawaha, and Withlacoochee River Basins
Low-Flow and Flood-Frequency Statistics
Regulation of Lake Levels
Temporal Trends in Lake Levels
Factors Controlling Lake Levels
Local water managers usually rely on information produced at the State and regional scale to make water-resource management decisions. Current assessments of hydrologic and water-quality conditions in Polk County, Florida, commonly end at the boundaries of two water management districts (South Florida Water Management District and the Southwest Florida Water Management District), which makes it difficult for managers to determine conditions throughout the county. The last comprehensive water-resources assessment of Polk County was published almost 40 years ago. To address the need for current countywide information, the U.S. Geological Survey began a 3?½-year study in 2002 to update information about hydrologic and water-quality conditions in Polk County and identify changes that have occurred.
Ground-water use in Polk County has decreased substantially since 1965. In 1965, total ground-water withdrawals in the county were about 350 million gallons per day. In 2002, withdrawals totaled about 285 million gallons per day, of which nearly 95 percent was from the Floridan aquifer system. Water-conservation practices mainly related to the phosphate-mining industry as well as the decrease in the number of mines in operation in Polk County have reduced total water use by about 65 million gallons per day since 1965.
Polk County is underlain by three principal hydrogeologic units. The uppermost water-bearing unit is the surficial aquifer system, which is unconfined and composed primarily of clastic deposits. The surficial aquifer system is underlain by the intermediate confining unit, which grades into the intermediate aquifer system and consists of up to two water-bearing zones composed of interbedded clastic and carbonate rocks. The lowermost hydrogeologic unit is the Floridan aquifer system. The Floridan aquifer system, a thick sequence of permeable limestone and dolostone, consists of the Upper Floridan aquifer, a middle semiconfining unit, a middle confining unit, and the Lower Floridan aquifer. The Upper Floridan aquifer provides most of the water required to meet demand in Polk County.
Data from about 300 geophysical and geologic logs were used to construct hydrogeologic maps showing the tops and thicknesses of the aquifers and confining units within Polk County. Thickness of the surficial aquifer system ranges from several feet thick or less in the extreme northwestern part of the county and along parts of the Peace River south of Bartow to more than 200 feet along the southern part of the Lake Wales Ridge in eastern Polk County. Thickness of the intermediate aquifer system/intermediate confining unit is highly variable throughout the county because of past erosional processes and sinkhole formation. Thickness of the unit ranges from less than 25 feet in the extreme northwestern part of the county to more than 300 feet in southwestern Polk County. The altitude of the top of the Upper Floridan aquifer in the county ranges from about 50 feet above National Geodetic Vertical Datum of 1929 (NGVD 29) in the northwestern part to more than 250 feet below NGVD 29 in the southern part.
Water levels in the Upper Floridan aquifer fluctuate seasonally, increasing during the wet season (June through September) and decreasing during the rest of the year. Water levels in the Upper Floridan aquifer also can change from year to year, depending on such factors as pumpage and climatic variations. In the southwestern part of the county, fluctuations in water use related to phosphate mining have had a major impact on ground-water levels. Hydrographs of selected wells in southwestern Polk County show a general decline in water levels that ended in the mid-1970s. This water-level decline coincides with an increase in water use associated with phosphate mining. A substantial increase in water levels that began in the mid-1970s coincides with a period of decreasing water use in the county.
Despite reductions in water use since 1970, however, over the long term, the increase in pumping in and near Polk County has resulted in a decline of the potentiometric surface of the Upper Floridan aquifer across much of the county since predevelopment times. Based on the difference between the estimated predevelopment potentiometric-surface map and water levels in wells measured in May and September and averaged from 2000 to 2004, water-level declines range from zero in the northwestern part of the county to as much as 40 feet in the southwestern part.
Water samples collected from 130 wells were used to characterize ground-water quality in Polk County. Samples from 53 wells and 1 spring in Polk and adjacent counties were collected for this project by the U.S. Geological Survey and analyzed for common inorganic constituents and nutrients. Concentrations of total dissolved solids, sulfate, and chloride in water samples from the surficial and intermediate aquifer systems generally were below State and Federal drinking-water standards. Nitrate concentrations, however, were as high as 26 milligrams per liter (mg/L) in samples from the surficial aquifer system along the Lake Wales Ridge. The application of fertilizers related to citrus farming is a likely source of nitrate to the ground water in this area.
Inorganic constituent concentrations in water from the Floridan aquifer system generally were below State and Federal drinking-water standards. Water from the Upper Floridan aquifer in most of Polk County is hard (hardness ranging from 70 to 290 mg/L), and has a dissolved-solids concentration of less than 500 mg/L. Chloride concentrations in water from the Upper Floridan aquifer range from 4.2 to 61 mg/L, and sulfate concentrations range from about 0.2 to 44 mg/L. In contrast to results from the surficial aquifer system, nitrate concentrations in the Upper Floridan aquifer generally were low (less than 0.02 mg/L) and exceeded 1.0 mg/L in only three wells.
Temporal trends in streamflow in the Peace River were updated in this study using data through 2003. The analyses also were expanded by analyzing trends over a wide range of the hydrologic regime and over several multidecadal periods. Results indicate that annual minimum to 70th percentile streamflows in the Upper Peace River began to decline in the 1950s, and this decline has persisted to 2003. Results also showed a statistically significant decrease in annual minimum streamflows in the Peace River at Bartow from 1974-1993. This decrease may be due to elimination of wastewater discharges to the stream during the mid-1980s.
Temporal trends in lake levels were analyzed to describe long-term trends (1960-2003) and trends of a shorter duration (1990-2003). About 90 percent of the lakes had no change in water levels from 1990-2003. Five of the lakes had an increasing trend in water levels. The increase in water levels likely is not due to increased rainfall. Annual totals at nearby rainfall stations had no significant trends during this period.
Spechler, R.M., and Kroening, S.E., 2007, Hydrology of Polk County, Florida: U.S. Geological Survey Scientific Investigations Report 2006-5320, 114 p.
U.S. Geological Survey
Florida Integrated Science Center
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Orlando, FL 32826
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