The Floridan aquifer system consists of the Upper and Lower Floridan aquifers separated by the middle confining unit. The middle confining unit and the Lower Floridan aquifer in west-central Florida generally contain highly mineralized water. The water-bearing units containing freshwater are herein referred to as the Upper Floridan aquifer. The Upper Floridan aquifer is the principal source of water in the Southwest Florida Water Management District (SWFWMD) and is used for major public supply, domestic use, irrigation, and brackish-water desalination in coastal communties (Southwest Florida Water Management District, 2000).
This map report shows the potentiometric surface of the Upper Floridan aquifer measured in September 2005. The potentiometric surface is an imaginary surface, connecting points of equal altitude to which water will rise in a tightly cased well that taps a confined aquifer system (Lohman, 1979). This map represents water-level conditions near the end of the wet season, when ground-water levels usually are at an annual high and withdrawals for agricultural use typically are low. The cumulative average rainfall of 55.19 inches for west-central Florida (from October 2004 through September 2005) was 2.00 inches above the historical cumulative average of 53.19 inches (Southwest Florida Water Management District, 2005). Historical cumulative averages are calculated from regional rainfall summary reports (1915 to the most recent calendar year) and are updated monthly by the SWFWMD.
This report, prepared by the U.S. Geological Survey (USGS) in cooperation with the SWFWMD, is part of a semiannual series of Upper Floridan aquifer potentiometric-surface map reports for west-central Florida. Potentiometric-surface maps have been prepared for January 1964, May 1969, May 1971, May 1973, May 1974, and for each May and September since 1975. Water-level data are collected in May and September each year to show the approximate annual low and high water-level conditions, respectively. Most of the water-level data for this map were collected by the USGS during September 19-23, 2005. Suppplemental water-level data were collected by other agencies and companies. A corresponding potentiometric-surface map was prepared for areas east and north of the SWFWMD boundary by the USGS office in Altamonte Springs, Florida (Kinnaman, 2006). Most water-level measurements were made during a 5-day period; therefore, measurements do not represent a "snapshot" of conditions at a specific time, nor do they necessarily coincide with the seasonal low water-level condition.
Water levels in about 86 percent of the wells measured in September 2005 were lower than the September 2004 water levels (Blanchard and Seidenfeld, 2005). Data from 400 wells indicate that the September 2005 water levels ranged from about 21 feet below to about 7 feet above the September 2004 water levels (fig. 1). The largest water-level declines occurred in south-central Hillsborough County and eastern Manatee County. The largest water-level rises occurred in east-central Sumter County, northeast Levy County, and northwest Marion County (fig. 1).
Water levels in about 86 percent of the wells measured in September 2005 were higher than the May 2005 water levels (Ortiz and Blanchard, 2005). Data from 398 wells indicate that the September 2005 water levels ranged from about 7 feet below to 10 feet above the May 2005 water levels. The largest water-level decline was in north-central Pasco County and the largest rise in water levels was in west-central Hardee County.
Blanchard, R.A., Seidenfeld, A.V., 2005, Potentiometric surface of the Upper Floridan aquifer, west-central Florida, September 2004: U.S. Geological Survey Open-File Report 2005-1222, 1 sheet.
Kinnaman, S.L., 2006, Potentiometric surface of the Upper Floridan aquifer in the St. Johns River Water Management District and vicinity, September 2005: U.S. Geological Survey Open-File Report 2006-1108, 1 sheet.
Lohman, S.W., 1979, Ground-water hydraulics : U.S. Geological Survey Professional Paper 708, 72 p.
Ortiz, A.G., and Blanchard, R.A., 2006, Potentiometric surface of the Upper Floridan aquifer, west-central Florida, May 2005: U.S. Geological Survey Open-File Report 2006-1009, 1 sheet.
Southwest Florida Water Management District, 2000, Aquifer characteristics within the Southwest Florida Water Management District: Brooksville, Fla. 123 p.
Southwest Florida Water Management District, 2005, Hydrologic conditions for the month of May 2005: Brooksville, Fla., 79 p.
Ortiz, A.G., and Blanchard, R.A., 2006, Potentiometric surface of the Upper Floridan aquifer,
west-central Florida, May 2005: U.S. Geological Survey Open-File Report 2006-1009, 1 sheet.
For more information, contact:
U.S. Department of the Interior
U.S. Geological Survey
10500 University Center Dr.
Tampa, FL 33612-6427