Evaluation of Methods to Determine Ground-water Seepage Below Levee 30, Miami-Dade County Florida

Ground-water flow models were 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 were obtained from: (1) previous studies conducted in the area, (2) analysis of a geologic core and geophysical logs from a new monitor well 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 measurement made at several locations under varying conditions in the Levee 30 canal, and (6) vertical seepage fluxes between surfacewater and groundwater in Water Conservation Area 3B obtained from seepage meters.

Determining the volume of water seeping from the water-conservation areas to the underlying aquifers is important in managing water levels in the conservation areas and freshwater deliveries to Everglades National Park. An accurate water budget to meet the competing natural and anthropogenic needs cannot be determined without this information. From Water Conservation Area 3B, water seeps into the Biscayne aquifer, which is about 80 feet thick directly beneath Levee 30 and thickens to the east, and flows relatively fast (due to high permeability of the aquifer) toward the urban and agricultural areas to the east. Water is also discharged to the canal along the eastern part of Levee 30. The rate of discharge is controlled by structures at the northern and southern ends of the canal. This seepage to the aquifer and canal discharge of water are 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 has resulted in higher ground-water seepage and canal discharge, reducing flows to the south in the water-conservation area. As a result, Levees 67A and 67C were constructed 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. Water managers want to restore predevelopment flow conditions for the Everglades to survive, while also taking into consideration the urban and agricultural needs. The objective of this project was to evaluate approaches for quantifying ground-water seepage beneath Levee 30. The accounting of all significant hydrologic inflows and outflows to the Everglades ecosystem of the south Florida mainland is a key element of the South Florida Ecosystem Program.