Preliminary Assessment
of Injection, Storage and Recovery of Freshwater in the Lower Hawthorn Aquifer,
Cape Coral, Florida
Water-Resources Investigations Report 94-4121
By Vicente Quinones-Aponte and Eliezer
J. Wexler
ABSTRACT
A
preliminary assessment of subsurface injection, storage, and recovery of
fresh canal water was made in the naturally brackish lower Hawthorn aquifer
in Cape Coral, southwestern Florida. A digital modeling approach was used
for this preliminary assessment, incorporating available data on hydrologic
conditions, aquifer properties, and water quality to simulate density-dispersive
transport of a conservative ground-water solute (Chloride ion).
A baseline simulation
was used as a reference to compare the effects of changing various operational
factors on the recovery efficiency. A recovery efficiency of 64 percent
was estimated for the baseline simulation. Based on the model, the recovery
efficiency increases if the injection rate and recovery rates are increased
and if the ratio of recovery rate to injection rate is increased. Recovery
efficiency decreases if the amount of water injected is increased; slightly
decreases if the storage time is increased; is not changed significantly
if the water is injected to a specific flow zone; increases with successive
cycles of injection, storage, and recovery; and decreases if the chloride
concentrations in either the injection water or native aquifer water are
increased. In several hypothetical tests, the recovery efficiency fluctuated
between 22 and about 100 percent.
Two successive cycles
could bring the recovery efficiency from 60 to about 80 percent. Inter-layer
solute mass movement across the upper and lower boundaries seems to be the
most important factor affecting the recovery efficiency. A sensitivity analysis
was performed applying a technique in which the change in the various factors
and the corresponding model responses are normalized so that meaningful
comparisons among the responses could be made. The general results from
the sensitivity analysis indicated that the permeabilities of the upper
and lower flow zones were the most important factors that produced the greatest
changes in the relative sensitivity of the recovery efficiency. Almost equally
significant changes occurred in the relative sensitivity of the recovery
efficiency when all porosity values of the upper and lower flow zones and
the leaky confining units and the vertical anisotropy ratio were changed.
The advective factors
are the most important in the Cape Coral area according to the sensitivity
analysis. However, the dispersivity values used in the model were extrapolated
from studies conducted at the nearby Lee County Water Treatment Plant, and
these values might not be representative of the actual dispersive characteristics
of the lower Hawthorn aquifer in the Cape Coral area.
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