Geohydrology and 1985 water withdrawals of the aquifer systems in southwest Florida, with emphasis on the intermediate aquifer system

Abstract

In a 4,700-square-mile area of southwest Florida, principal hydrogeologic units are the surficial aquifer system, the intermediate aquifer system, and the Floridan aquifer system. The thickness of the surficial aquifer system ranges from 25 to 250 feet, and transmissivity ranges from about 1,100 to about 8,000 feet squared per day. The intermediate aquifer system includes all water-bearing units and confining units between the overlying surficial aquifer system and the underlying Floridan aquifer system. The top of the intermediate aquifer system ranges from more than 100 feet below sea level in Highlands County to more than 100 feet above sea level in central Polk County. Thickness ranges from less than 100 feet to more than 800 feet, and transmissivity ranges from less than 200 to about 13,000 feet squared per day. Leakance of the confining units ranges from 1x10 7 to 4x10 4 foot per day per foot. The Floridan aquifer system consists of the Upper and Lower Floridan aquifers separated by a tight middle confining unit. Transmissivity of the Upper Floridan aquifer in the study area ranges from about 30,000 feet squared per day at the gulf coast where the freshwater zone is thin to about 400,000 feet squared per day in eastern De Soto and Hardee Counties. The altitude of the potentiometric surface of the intermediate aquifer system in September 1985 ranged from 120 feet above sea level in Polk County to less than 20 feet above sea level near the coast. In the northern part of the study area, water levels are higher in the intermediate aquifer system than water levels in the underlying Upper Floridan aquifer. The hydraulic gradient reverses in the southern part of the area. In 1985, in the study area, an estimated 808 million gallons per day of freshwater was withdrawn from the surficial and intermediate aquifer systems and Upper Floridan aquifer for irrigation, public and rural supply, and industrial use. Of this total, an estimated 68.9 million gallons per day was withdrawn from the intermediate aquifer system.