The Floridan aquifer system consists of limestone, dolomite, and calcareous sand; and is one of the most productive groundwater reservoirs in the United States. Regionally, the Floridan aquifer system has been divided into the Upper and the Lower Floridan aquifers (Miller, 1986). In Georgia, about 655 million gallons of water per day is pumped from the Upper Floridan aquifer, mostly for agricultural and industrial use (Pierce and Kunde 1,1990). A potentiometric surface shows the level to which water would rise in tightly-cased wells that fully penetrate an aquifer. This report shows the potentiometric surface of the Upper Floridan aquifer in Georgia, and adjacent parts of Alabama, Florida, and South Carolina. The Upper Floridan underlies most of the Coastal Plain physiographic province in Georgia. The Upper Floridan is confined, except where rocks comprising the aquifer crop out at land surface. In some areas, the potentiometric surface is hijgh enough to produce flowing wells. Water-level and water-pressure measurements in more than 1,070 wells in Georgia and adjacent parts of Alabama, South Carolina, and Florida during May 9 to June 3,1990, were used to construct the potentiometric surface of the Upper Floridan aquifer (fig. 1). The potentiometric surface was mapped by determining the altitude of water levels in a network of wells (fig. 1), and is shown on the map by contours that connect points of equal altitude. Maps showing the potentiometric surface of the Upper Floridan aquifer in Georgia, and adjacent states of Alabama, Florida, and South Carolina, are prepared periodically by the U.S.

An investigation to document groundwater levels and stream base flow in the lower Chattahoochee-Flint and western and central Aucilla-Suwanee-Ochlockonee River basins during low-flow conditions was conducted by the U.S. Geological Survey in November 2008. During most of 2008, moderate to severe drought conditions prevailed throughout southwestern Georgia. Groundwater levels were below median daily levels throughout most of 2008; however, in some wells, groundwater levels rose to median daily levels by November. Discharge in most of the streams in the study area also had risen to median levels by November. The potentiometric surface of the Upper Floridan aquifer was constructed from water-level measurements collected in 21 counties from 376 wells during November 1-10, 2008. The potentiometric surface indicates that groundwater in the study area generally flows to the south and toward streams except in reaches discharging to the Upper Floridan aquifer. The degree of connection between the Upper Floridan aquifer and streams decreases east of the Flint River where the overburden is thicker. Decreased connectivity between ground and surface water is evident from the stream-stage altitudes measured in November 2008 east of the Flint River, which are not similar to water-level altitudes measured in the Upper Floridan aquifer. Stream-stage measurements were collected at 111 sites-26 U.S. Geological Survey streamgaging sites and 85 additional synoptic sites without gages. Streamflow measurements were made at 87 of the sites during November 2008 and were used to estimate base flow. The measurements indicate that stream reaches range from losing up to 10 cubic feet per second to gaining up to 4,559 cubic feet per second; five stream reaches were determined to be losing stream reaches. Of the 11 stream reaches in the Alapaha River subbasin, 7 were dry when measured in November 2008.

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