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Fault-Controlled Freshwater Lenses within the Saline 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 lenses, locally known as bubbles, each surrounded by saline water. These lenses are located within two limestone intervals or flow zones within the Suwannee-Ocala and the Avon Park Formations, respectively. Well logs confirm the presence of major faulting in the immediate area. This study considers that the freshwater recharge to these lenses migrates along these faults from updip inland areas. The water contained in each of the lenses 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 lenses. Therefore, it is proposed that freshwater is migrating along faults into the lenses of each zone. A previous study in 1985 proposed that the lenses were isolated with no recharge and estimated the yield life of the Suwannee-Ocala zone to be 16 years for the Evans Pines and Ballard lenses. Today, ten 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.
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