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hydrogeology of a dynamic system in the florida keys: a tracer experiment >
results -- tracer experiments
Part 2: Results -- Tracer Experiments
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Infrared-enhanced aerial photograph of
our study site. The two yellow stars depict the locations of our two well
cluster sites. This photo shows a section of Key Largo, upper Florida
Keys, with Florida Bay to the north (top of photo) and Atlantic Ocean to
the south (bottom of the photo). (Click on photo for larger version.) |
| The diagram at the right depicts what
the well cluster arrangement looks like after completion of all piezometers
in Florida Bay and in the Atlantic Ocean. The well clusters were completed
so that all of the piezometers were 100 ft (30.3 m) away from the central
well. The central well would become the location for the injection of
dye to trace the natural gradient of groundwater flow. Fluorescein dye
was injected in the shallow, central piezometers at both sites and Rhodamine
WT injected in the deep, central piezometers at both sites. The central
well for the ocean side well cluster is approximately 100 m offshore and
the central well at the bayside well cluster is approximately 75 m offshore.
Benchmarks were installed onshore on both sides of Key Largo and position
and elevation determined using DGPS. Two wells at each cluster were later
leveled in using the benchmarks as a reference point. |
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Oceanside Well Cluster (OSWC) |
(Click on image above for larger
version.) |
Based on first arrival of Fluorescein
dye the net direction of flow was to the southeast (toward the reef tract)
with a velocity of 0.61 m/d. A small component of flow (dispersive) was
detected in piezometers running parallel to the shoreline. First arrival
times were used in calculating all velocities because not all breakthrough
curves were sigmoidal.
A second injection of Fluorescein on day
332 did not result in a second peak as expected. One possible reason is
that some insoluble portion of Fluorescein, due to incomplete mixing, plugged
up the well screen during injection which didn't allow enough dye to enter
the formation. Therefore, insufficient concentrations were present above
the detection limit of the fluorometer.
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Fluorescein
breakthrough curves
(Click on graphs to view larger version.)
deep: 13.6
m (__ )
and
shallow: 6.1 m (- - -
)
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Rhodamine WT breakthrough curves (below)
for the eight piezometer nests, and velocity vector diagrams (right) based
on first arrival of dye from the breakthrough curves.
Rhodamine was detected in the shallow
(B) piezometer at wells 5 and 6 indicating a component of vertical transport.
At well 6A&B, Rhodamine was detected simultaneously. A flow rate was
calculated to be 0.51 m/d in the southeast direction toward the reef tract.
The spike at the tail end of the breakthrough curves corresponds to the
second injection of dye on day 238. Weather conditions during the second
injection resulted in flow in the opposite direction (toward Fl. Bay) at
rates as high as 1.5 m/d at 2A&B. This suggests that weather plays
a major role in flow of ground water at this site. |
 |
Rhodamine WT
breakthrough curves
(Click on graphs to view larger version.)
deep: 13.6
m (__ )
and
shallow: 6.1 m (- - -
)
|
Bayside Well Cluster (OSWC) |
 |
Groundwater flow velocity at the Bayside
Well Cluster was greater than velocity at the Oceanside Well Cluster.
Fluorescein was detected in 12 days at 2B and 3B which indicates a flow
velocity of 2.5 m/d in the southeast direction toward the reef tract.
A component of flow to the north was also detected with velocity of 0.36
m/d. This was most likely a dispersive flow induced by tidal pumping.
A spike from the second injection of Fluorescein
on day 182 was observed at 6B. This suggests that weather plays a role
in groundwater flow and is similar to the results from the second Rhodamine
injection at OSWC. Flow was in the opposite direction (toward Fl. Bay)
after the second injection with a velocity of 2.0 m/d. |
Fluorescein
breakthrough curves
(Click on graphs to view larger version.)
deep: 13.6
m ( __)
and
shallow: 6.1 m ( - - -
)
|
Rhodamine was detected at the same wells
and at similar arrival times as was observed during the Fluorescein test.
The only exception is that Rhodamine appeared more frequently and in higher
concentrations in the shallow piezometers. Whereas Fluorescein was found
in few deep wells and, if present, in very low concentrations. The greatest
flow velocity for Rhodamine was 2.5 m/d at 2B & 3B.
The second injection on day 182 resulted
in a similar outcome as observed in the Fluorescein results. Dye was picked
up at 6A&B in 10 days which results in a velocity of 3.0 m/d. Again
this flow was not observed in the first dye injection, and was the result
of wind blowing consistently out of the east at 20 - 25 mph. |
 |
Rhodamine WT
breakthrough curves
(Click on graphs to view larger version.)
deep: 13.6
m (__ )
and
shallow: 6.1 m (- - -
)
|
Next: Tidal Pumping and Meteorological Influences
on Groundwater Flows
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