Analysis of Tests of Subsurface Injection, Storage, and Recovery of Freshwater
in the Lower Floridan Aquifer, Okeechobee County, Florida
By Vicente Quinones-Aponte, Kevin
Kotun, and Joseph F. Whitley
ABSTRACT
A series of freshwater
subsurface injection, storage, and recovery tests were conducted at an injection-well
site near Lake Okeechobee in Okeechobee County, Florida to assess the recoverability
of injected canal water from the Lower Floridan aquifer. At the study site,
the Lower Floridan aquifer is characterized as having four local, relatively
independent, high-permeability flow zones (389 to 398 meters, 419 to 424
meters, 456 to 462 meters, and 472 to 476 meters below sea level). Four
subsurface injection, storage, and recovery cycles were performed at the
Lake Okeechobee injection-well site in which volumes of water injected ranged
from about 387,275 to 1,343,675 cubic meters for all the cycles, and volumes
of water recovered efficiency for successive cycles 2 and 3 increased from
22 percent to 36 percent and is expected to continue increasing with additional
cycles.
A comparison of
chloride concentration breakthrough curves at the deep monitor well (located
about 171 meters from the injection well) for cycles 1, 4 and test no.4
(from a previous study) revealed unexpected findings. One significant result
was that the concentration asymptote, expected to be reached at concentration
levels equivalent or close to the injected water concentration, was instead
reached at higher concentration levels. The injection to recovery rate ratio
might affect the chloride concentration breakthrough curve at the deep monitor
well, which could explain this unexpected behavior. Because there are four
high-permeability zones, if the rate of injection is smaller than the rate
of recovery (natural artesian flow), the head differential might not be
transmitted through the entire open wellbore, and injected water would probably
flow only through the upper high-permeability zones. Therefore, observed
chloride concentration values at the deep monitor well would be higher than
the concentration of the injected water from the different high-permeability
zones.
A generalized digital
model was constructed to simulate the subsurface injection, storage, and
recovery of freshwater in the Lower Floridan aquifer at the Lake Okeechobee
injection-well site. The model was constructed using a modified version
of the Saturated-Unsaturated TRAnsport code (SUTRA),
which simulates variable-density advective-dispersive solute transport and
variable-density ground-water flow. Satisfactory comparison of simulated
to observe dimensionless chloride concentrations for the deep monitor well
were obtained when using the model during the injection and recovery phases
of cycle 1, but not for the determination of the recovery phase of cycle
1 even after several attempts. This precluded the determination of the recovery
efficiency values by using the model.
The unsatisfactory
comparisons of simulated to observe dimensionless chloride concentrations
for the injection well and failure of the model to represent the field data
at this well could be due to the characteristics of the Lower Floridan aquifer
(at the local scale), which is cavernous or conduit in nature. To test this
possibility, Reynolds numbers were estimated at varying distances from the
injection well, taking into consideration two aquifer types or conceptual
systems, porous media and cavernous. For the porous media conceptual system,
the Reynolds numbers were greater than 10 at distances less than 1.42 meters
from the injection well. Thus, application of Darcy's law to ground-water
flow might not be valid at this distance. However, at the deep monitor well
(171 meters from the injection well), the Reynolds number was 0.08 which
is indicative of laminar porous media flow. For the cavernous conceptual
system, the Reynolds numbers were greater than 2,000 at distances less than
1,000 meters from the well. This number represents the upper limit of laminar
flow, which is the fundamental assumption for the application of Darcy's
law to free flow.
Results from the
study suggest that to simulate recovery efficiency for the Lower Floridan
aquifer at the Lake Okeechobee injection-well site might require the application
of a free-flow type model (conduit flow or fracture flow). This type of
model may produce a more realistic representation of the actual fluid motion
in the Lower Floridan aquifer and could provide appropriate estimates of
the recovery efficiency.
(The entire report
is available below.)
