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Geology and Tidal Pumping in the Florida Keys

SHINN, E. A., C. D. REICH, T. D. HICKEY, and A. B. TIHANSKY

Tidal pumping, combined with thin (1-10 cm) calcrete layers capping subaerial unconformities and Holocene carbonate mud control both vertical and lateral movement of ground water in the Florida Keys, Florida Bay, and reef tract. Ground water pressure fluctuations, as measured in piezometers installed in Pleistocene limestone to depths ranging from 2 to 20 m, are caused by tidal pumping. Tidal pumping decreases with depth and is present on both the Atlantic and Florida Bay sides of the Keys. Ground water pressure in Florida Bay, where lunar tides are minimal, is tuned to tidal fluctuations on the Atlantic side of the Keys, attesting to the extremely permeable nature of the limestone. Thus, when the tide is low on the Atlantic side of the Keys, ground water pressure under Florida Bay is negative. The opposite occurs during high tide. Dye tracer studies in well clusters on both sides of the Keys show that these alternating positive and negative pressures, as much as 24 centimeters relative to water surface, and the relatively higher sea level in Florida Bay force net flow seaward (i.e., "down hill") through the Keys limestone. Conservative estimates of ground water flow rates range from 0.5 to 2 m/day.

Contaminants, namely NO[2], NO[3], NH[4], and fecal bacteria are present in both onshore and offshore ground water. Sources of these contaminants in the Florida Keys include >25,000 septic tanks, >5,000 cesspools, and >700 shallow, sewage injection wells.

AAPG Search and Discovery Article #91021©1997 AAPG Annual Convention, Dallas, Texas.