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Conclusions

Based on the research documented in this report, general conclusions about the discharge of ground water to Biscayne Bay are as follows:

• Based on the results from a regional-scale, variable-density, ground-water flow model, the average rate of fresh ground-water discharge to Biscayne Bay seems to be about 6 percent of the average rate of surface-water discharge to the bay. During dry periods, however, monthly average rates of fresh ground-water discharge can exceed surface-water discharge, which suggests that ground-water discharge to Biscayne Bay may be dominant during certain times.
• Temporal variations in average monthly ground-water discharge are caused by relatively large variations in sea-level elevation as well as seasonal variations in the height of the water table.
• Fresh ground-water discharge to Biscayne Bay is spatially variable. Fresh ground-water discharge to the tidal portions of the Miami and Coral Gables Canals probably is similar in magnitude to the coastal ground-water discharge to Biscayne Bay. Nearly all of the fresh ground-water discharge directly to Biscayne Bay occurs north of structure S-123. Ground-water discharge rates south of this structure are minimal because low-lying areas adjacent to the bay suppress the elevation of the water table and reduce the hydraulic gradient toward the coast.
• The field data and results from the numerical models suggest that ground-water discharge to Biscayne Bay is confined to a narrow band adjacent to the coast that probably is less than 1 km wide. The ground-water discharge band is probably about 1 km wide near the Deering Estate. North of the Deering Estate, the discharge band is probably hundreds of meters wide; whereas south of the Deering Estate, it is probably only tens of meters wide, if present at all. The concept of a narrow discharge band is most meaningful in a regional context because it is based on field data at three discrete transects and the results from numerical models that are based on the assumption of an equivalent porous medium. Therefore, at local-scales where preferential ground-water flow pathways or conduits may be present, the concept of the narrow discharge band may not apply.

Results from this study have a large degree of uncertainty, which is why conclusions are reported in relative terms rather than absolute values. The large degree of uncertainty is primarily attributed to two reasons: (1) ground-water discharge rates were not directly measured, and (2) ground-water discharge rates were indirectly estimated with a variable-density ground-water flow model. Attempts to use seepage meters to directly measure discharge rates were unsuccessful. Future attempts to measure ground-water discharge to Biscayne Bay would probably benefit from trying to incorporate some direct measurement technique of discharge as well as measurements of salinity in the bay. Variable-density models require the accurate simulation of transport processes and the accurate simulation of flow processes. Errors in this type of model are compounded because flow and transport are coupled processes. Although these uncertainties limit the reliability of the simulated discharge estimates, the estimates probably are the best available because they were derived with a physics-based ground-water flow model that includes the variable-density effects known to influence coastal ground-water flow.

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