To manage water levels in the water conservation areas and freshwater deliveries to Everglades National Park, it is important to determine the volume of water seeping from the water conservation areas to the underlying aquifers. An accurate water budget to meet the competing natural and anthropogenic needs cannot be determined without this information. The U.S. Geological Survey is presently conducting a study to evaluate methods for quantifying these seepage losses. The study site is located along the 14-mile long Levee 30 and adjacent canal in north-central Dade County. Completed in 1954, Levee 30 is part of the eastern boundary of Water Conservation Area 3B. From this water conservation area, water seeps into the Biscayne aquifer, which is about 80 feet thick directly beneath Levee 30 and thickens to the east. Water flows relatively fast in the aquifer (due to high permeability of the aquifer) toward the urban and agricultural areas to the east. This seepage to the aquifer from Water Conservation Area 3B is critical for water-supply wells to the east and for preventing the inland movement of saltwater from the coast. However, lowering of ground-water levels to the east resulted in greater ground-water flow eastward from Water Conservation Area 3B and reduction of surface-water flows to the south. As a result, Levees 67A and 67C were constructed west of Levee 30 to direct water southward toward the central region of Everglades National Park. This water-management scheme has been effective in delivering water to the southwest; however, it reduced the flow to the southeast (northeastern part of Everglades National Park). The altering of historical flow directions and water-level durations has caused significant adverse effects to parts of the Everglades ecosystem.

Ground-water flow models are being developed to calculate a water budget, including seepage losses, for a transect perpendicular to Levee 30. Data required for input to and calibration of the models have been obtained from: (1) previous studies conducted in the area, (2) analysis of a geologic core and geophysical logs from new monitor wells drilled along the transect, (3) ground-water-level data from monitor wells along the transect, (4) surface-water stage data in Water Conservation Area 3B and in the Levee 30 canal, (5) discharge measurements made at several locations under varying conditions in the Levee 30 canal, and (6) vertical seepage rates obtained from seepage meters installed in Water Conservation Area 3B under varying hydrologic gradients.

A continuous geologic core from land surface to a depth of 78 feet was obtained during the drilling of a monitor well completed in February 1995. The surface soils consist of about 5 feet of Everglades peat, with the remaining 73 feet consisting almost entirely of very porous limestone and shells. Tests performed on 10 plugs from the core indicated porosities as high as 45 percent and permeabilities as high as 9,500 millidarcys; both values are indicative of the extremely high permeabilities associated with the Biscayne aquifer. Of particular interest was a thin, very hard, impermeable limestone layer at 7 feet below land surface with a very low porosity (less than 5 percent) and very low permeability (less than 0.001 millidarcy). This layer is believed to be areally extensive and constitutes a semiconfining layer that retards the seepage of water from Water Conservation Area 3B into the underlying Biscayne aquifer.

Twenty-one continuous recording ground-water-level monitor wells were installed along the transect, running about 500 feet both east and west of Levee 30. The wells are located in six different clusters; each cluster has two to five wells, with depths ranging from 10 to 80 feet below land surface. Continuous surface-water-level (stage) recorders were also installed along the transect, one in Water Conservation Area 3B and one in the Levee 30 canal. Data were collected from February to December 1996 to obtain information for both wet- and dry-season conditions. The data are being used to select boundary conditions for the ground-water flow models and to calibrate the models.

Initial data indicate a significant difference between the stage in Water Conservation Area 3B and the water levels in the Biscayne aquifer--as much as 0.5-foot head difference between the water conservation area wetlands and the aquifer during periods of high water. This ponding of the surface water is believed to be the result of the thin, low-permeability, limestone layer located near the top of the aquifer. The head difference between the water conservation area and the canal is even greater, with an average difference of 0.84 foot for the period of data collection. Data from the vertical seepage meters show the effects of these head differences. An increased head difference, a result of the lowering of the canal stage when the gate to the south is open, results in the vertical seepage flux increasing from 0.04 to 0.09 foot per day at the meter located 500 feet west of the levee.

Water in the aquifer flows into and beneath the canal along the eastern side of Levee 30. The rate of ground-water seepage into the canal is controlled by the head difference between the aquifer and the canal. Structures at the northern and southern ends of the canal are used to discharge water from the canal, lowering the canal stage and increasing the discharge rate from the aquifer into the canal. A layer of fine sediments (at the bottom of the canal), measured to be at least 2 feet thick, retards seepage into the canal from the underlying aquifer. Thus, based on the head differences between the aquifer and the canal and the confining sediments at the bottom of the canal, most of the seepage into the canal appears to be from the uppermost part of the aquifer on the western side of the canal through the side of the canal.

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