Satellite image of South Florida.
Image shows Florida Everglades at the top of the picture; Florida Bay
with its anastimozing mudbanks and mud islands in the center; the arcuate
chain of islands that make up the Florida Keys; and the reef tract that
runs parallel to the Keys. Our study site is located in the upper right
hand side of the photograph.(Click on photo above for larger version.)
Fluorescein and rhodamine dyes and an inert
gas solution, sulfur hexafluoride (SF6), were injected into the shallow
subsurface to determine flow velocities and direction within the groundwater-flow
system in the upper Florida Keys. In the Florida Keys, ground water is
entirely saline or hypersaline with isolated freshwater/brackish-water
lenses on the larger islands. Ground water flows through a highly permeable
and porous Pleistocene limestone, known as the Key Largo Limestone. The
Key Largo Limestone is a coralline boundstone containing numerous primary
and secondary porosity features, making it a very transmissive unit. Estimates
of hydraulic conductivity for the Key Largo Limestone Formation range from
1,400 m/d (Vacher et al., 1992) to 8,800 m/d (Fish and Stewart, 1990).
Two 60.6-m-diameter underwater well clusters
were installed in the nearshore waters of the Florida Keys in approximately
0.6 to 2.4 m of water depth. One well cluster is located in Florida Bay
on the northwest side of Key Largo and the other is located directly across
the island to the southeast in the Atlantic Ocean. There are nine holes
to a depth of 13.6 m; eight are in a 30.3-m-radius around a central site.
Well clusters were designed with a deep and shallow well in a single borehole.
The deep piezometers (13.6 m) are completed with 1.5-m-long well screens
set in a quartz sand pack and capped with Portland cement. Shallow piezometers
(6.1 m) have a 1.5-m-long screen and are suspended within the open hole.
The wells are sealed with a cap of cement at the seafloor surface, leaving
approximately 25 to 35 cm of the wellhead exposed for subsequent sampling.
Caps are placed on each piezometer head to prevent sea flora and fauna
from inhabiting the wells.
Ground water from the piezometers was analyzed
for fluorescence using a filter fluorometer. Groundwater-flow velocities
were calculated from breakthrough curves. Flow velocities were as high
as 2 m/d in an easterly and southeasterly direction (i.e., toward the reef
tract). However, it was noted that velocity on the bay side was 3 to 5
times greater than on the ocean side of Key Largo. The drive for horizontal
groundwater flow in this region is thought to be the difference between
sea level in Florida Bay and the Atlantic Ocean. On average, Florida Bay
is 15 to 20 cm higher than the Atlantic Ocean. In the upper Florida Bay
region, astronomical tides are absent, but on the Atlantic side of Key
Largo, tides are semi-diurnal and have a range of approximately 1 m.<
Dye injected in the deep piezometer appeared
first in the shallow peripheral piezometers, indicating an upward transport
of ground water. This observation is the result of a phenomenon known
as tidal pumping. Tidal pumping is a measure of pressure head in underwater
piezometers relative to sea level. Piezometers in both Florida Bay and
the Atlantic Ocean directly reflect the pressure changes due to ebbing
or flooding tides. Also, deeper piezometers show a greater dampening of
the tidal signal than do the shallow piezometers. Pressure heads can exceed
±20 cm, with shallow piezometers having a slightly greater pressure
than the deeper piezometer in the same borehole, and negative head pressures
are generally greater than positive head pressures. The effects of the
pressure gradients result in a vertical component of groundwater flow.