Abstract: Fault-Controlled Freshwater Bubbles Within the Saline Floridan Aquifer of Coastal Brevard and Indian River Counties, Florida

Hugh J. Mitchell-Tapping

In southeast Brevard and northeast Indian River Counties along the eastern coastal barrier island area of Florida at depths below 100 m (300 ft) in the normally highly-saline Upper Floridan aquifer system, there are four freshwater bubbles or lenses, each surrounded by saline water. These bubbles are considered fault-controlled freshwater storage areas located within two zones: the Suwannee-Ocala and the Avon Park. Well logs confirm the presence of major faulting in the immediate area. The water contained in each of the bubbles shows similar chemical composition. Age-dating of both the saline and freshwater give values about 30,000 YBP (years before present). As the Floridan aquifer exhibits upward artesian pressure in this discharge coastal area, there can be no downward surface recharge to the bubbles. Therefore, it is proposed that freshwater is migrating along faults into the bubbles of each zone. A previous study in 1985 estimated the yield life of the Suwanne-Ocala zone was 16 years for the Evans Pines, Aquarina and Ballard bubbles. Today, 10 years later, there has been no deterioration of water quality or a major change in hydraulic head, even though there has been a large increase in the quantity used. Measured fluctuations in hydraulic head, usually occurring after major rainfalls updip, and water quality tests support the hypothesis of freshwater influx via faults.

AAPG Search and Discovery Article #90955©1995 GCAGS 45th Annual Meeting and Gulf Section SEPM, Baton Rouge, Louisiana