Continuous recording, ground-water-level monitor wells (21) 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.

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 a new monitor wells drilled along the transect, (3) ground-water-level data from monitor wells along the transect, (4) surface-water-stage data in WCA 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 WCA3B under varying hydrologic gradients.

Continuous recording, ground-water-level monitor wells (21) 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 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 conservation area and the canal is even greater, with an average difference of 0.84 feet 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 ft/day to 0.09 ft/day at the meter located 500 feet west of the levee.

Wet Season Water Level Contours And Flow Lines (September 16, 1996)

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Dry Season Water Level Contours And Flow Lines (May 18, 1996)

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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, which was 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, most of the seepage into the canal appears to be from the upper most part of the aquifer on the west side of the canal through the side of the canal.

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, therefore, would constitute a semiconfining layer retarding the seepage of water from Water Conservation Area 3B into the underlying Biscayne aquifer.

Geophysical logs were obtained from two monitor wells using electromagnetic induction and natural gamma tools. The resulting logs are useful for determining changes in rock types and water quality.

Collection of Vertical Seepage Data Using Seepage Meters

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Relation between seepage rate and head difference between surface-water and groundwater. [larger image]

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For more information contact:

Roy S. Sonenshein
U.S. Geological Survey
(305)717-5824
Email sunshine@usgs.gov

Related information:

SOFIA Project: Evaluation of Methods to Determine Groundwater Seepage Beneath Levee 30, Miami-Dade County, Florida

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