Electrical Resistvity Tracer Teststo Estimate Seepage Velocity In the Biscayne Aquifer, Miami-Dade Ccouty, Florida

The Biscayne Aquifer underlies most of Miami-Dade County, including the Everglades National Park (ENP), and is the primary drinking water supply for Miami.  Biscayne Aquifer porosity is characterized by interparticle voids, karstic vugs, conduits, and caverns with transmissivity values that range from <300,000 to > 1,000,000 ft2/day.   The high transmissivity and complex porosity of the aquifer have made traditional tracer tests difficult due to the difficulty in predicting flow paths well enough to site monitoring wells to detect the tracer.  Construction of a pilot slurry wall was proposed to restore historic flow regimes in Everglades National Park.  Prior to construction, baseline hydrogeologic tests were performed to characterize the Biscayne Aquifer at the construction site.  Two bromide tracer tests with surface resistivity surveys were conducted to better characterize ground water flow rate and direction.  The resistivity surveys were able to monitor tracer migration in the aquifer independent of the position of monitoring wells. 

Bromide tracer was introduced through a well with an open interval in the Biscayne Aquifer.  Resistivity measurements were made at frequent intervals along a series of profile lines around the injection well.   Reduction in electrical resistivity occurred as ion tracer travelled down gradient through complex heterogeneous aquifer matrix.  Seepage rate was measured using the down gradient tracer breakthrough time.  Break-through was observed along 3-D flow paths defined by resistivity contrast between native groundwater and tracer, as detected from surface to 70 ft below land surface.

Preliminary seepage flow rates from selected runs range between 130 and 230 ft/day.  Variations of seepage rates along the flow path may be related to changes in the throat size of the void systems.  The variations in measurements may reflect actual variations in flow through the aquifer and may be useful in estimating changes in the aperature of the flow conduit.