Open File Report 2006-1108

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Introduction

      This map depicts the potentiometric surface of the Upper Floridan aquifer in the St. Johns River Water Management District and vicinity for September 2005. Potentiometric contours are based on water-level measurements collected at 643 wells during the period September 12-28, near the end of the wet season. Some contours are inferred from previous potentiometric-surface maps with larger well networks. The potentiometric surface of the carbonate Upper Floridan aquifer responds mainly to rainfall, and more locally, to ground-water withdrawals and springflow. Potentiometric-surface highs generally correspond to topographic highs where the aquifer is recharged. Springs and areas of diffuse upward leakage naturally discharge water from the aquifer and are most prevalent along the St. Johns River. Areas of discharge are reflected by depressions in the potentiometric surface. Ground-water withdrawals locally have lowered the potentiometric surface. Ground water in the Upper Floridan aquifer generally flows from potentiometric highs to potentiometric lows in a direction perpendicular to the contours.

Summary of Hydrologic Conditions

      Measured values of the potentiometric surface ranged from 3 feet below NGVD 29 near Fernandina Beach, Florida to 130 feet above NGVD 29 in Polk County, Florida. The average water level of the network in September 2005 was about a foot higher than the average in May 2005 following near-average rainfall during the previous 12 months. Seasonal differences in network average water levels generally range from 4 to 6 feet. The below-average seasonal change in water levels is reflective of the above-average rainfall in 2004. For 583 wells with previous measurements, September 2005 levels ranged from about 11 feet below to about 10 feet above May 2005 water levels. For 585 wells with previous measurements, September 2005 levels ranged from about 10 feet below to about 11 feet above September 2004 water levels. The average water level of the network in September 2005 was about a foot lower than the average in September 2004.

Additional Reference

Long-term hydrographs of ground-water levels site: http://waterdata.usgs.gov/fl/nwis/gw