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Abstract: Hydrogeology of a Dynamic Marine System in a Carbonate Environment, Key Largo Limestone Formation, Florida Keys

U.S. Geological Survey, St. Petersburg, FL
U.S. Geological Survey, Tampa, FL

Tidal signal, wave amplitude, and wind pattern all combine to set up a unique and dynamic system that drives groundwater flow within the highly porous and permeable Key Largo Limestone. Pressure-head measurements and dye-tracer experiments in underwater piezometer clusters installed off the upper Keys have demonstrated that net groundwater flow occurs from the Florida Bay to the Atlantic Ocean.

Results from tracer tests, including fluorescein and rhodamine dye, and an inert gas, sulfur hexafluoride (SF6) solution, demonstrate that net groundwater flow occurs through the Keys with a velocity as high as 2 m/day. In addition, the direction of flow is congruent with data that indicates Florida Bay sea level is on average higher, 10 to 20 cm, than mean Atlantic Ocean sea level. The difference between bay and ocean sea level is the main driving force for groundwater flow beneath the upper Keys. However, sustained high easterly winds periodically depress the bay level and raise water level on the Atlantic side of the upper Keys. These conditions cause the groundwater flow direction to reverse, driving marine groundwater to flow at similar velocities from the Atlantic Ocean beneath Key Largo toward Florida Bay.

Tracer studies also indicate that dye injected in a deep (13.6-m) central well appears first in shallow (6.1 -m) peripheral wells 30 m away. Vertical groundwater movement and seepage into surface water may be attributed to tidal pumping observed as a pressure change in the piezometers. Seepage of nutrient-rich ground water into nearby surface water can have an impact on the diversity and health of nearshore marine environments.

AAPG Search and Discovery Article #90930©1998 AAPG Eastern Section, Columbus, Ohio