Injection of two dye tracers, fluorescein and rhodamine, as well as an inert gas solution, sulfur hexafluoride (SF₆ ), and subsequent monitoring in surrounding piezometers, exhibits a net submarine ground-water flow from Florida Bay to the Atlantic Ocean with velocities up to 2 meters per day. In addition, direction of flow based on tracer experiments is congruent with sea level information which indicates that Florida Bay is higher, 10 to 20 centimeters on average, than mean Atlantic Ocean sea level. The difference between bay and ocean sea level is the main driving force for ground-water flow beneath the upper Keys. However, sustained high easterly winds periodically depress the bay level, raise water level on the Atlantic side of the upper Keys, and drive marine ground water at similar velocities beneath Key Largo toward Florida Bay.

In addition, tidal pumping, a phenomenon observed in our piezometers, is the result of a difference in tides between the Atlantic Ocean and Florida Bay. Tracer studies indicate that dye injected in deep (13.6 meters) wells appear first in shallow (6.1 meters) wells 30 meters away. The vertical tracer movement may be attributed to tidal pumping. Vertical ground-water flow and, ultimately, seepage of nutrient-rich ground water into the surface water may have an impact on the diversity and health of the nearshore marine environment.

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