USING BULK RESISTIVITY TO PREDICT SPECIFIC CONDUCTANCE AND CHLORIDE CONCENTRATION IN THE BISCAYNE AQUIFER

The Biscayne Aquifer is a highly transmissive, heterogeneous limestone aquifer in southeastern Florida, which is impacted by saltwater intrusion. Chloride concentration and specific conductance are common indicators of saltwater intrusion. Variations in the electrical resistivity of a homogeneous aquifer can also be used to evaluate saltwater intrusion because these changes are mostly due to changes in groundwater chemical composition caused by this intrusion. Previous studies have used Archie’s equations or regression models to estimate specific conductance and chloride concentration using bulk resistivity measurements from borehole electromagnetic induction logs. Archie’s Equations depend on aquifer porosity, cementation and tortuosity, and there is variability in the electrical properties of the different major ions. Therefore, it is important to consider the influence of heterogeneities in both aquifer properties and groundwater chemical composition when developing and applying proxy relationships for estimating specific conductance and chloride from bulk resistivity.

The U.S. Geological Survey, in cooperation with Miami-Dade County, has an extensive saltwater intrusion monitoring network in the Biscayne Aquifer. This monitoring effort includes annual borehole electromagnetic logging and periodic measurements of chloride and specific conductance in approximately 70 wells, many of which have a greater than 20-year record. This presentation will summarize an ongoing study that is using this expansive data set to improve understanding of the relationships between bulk resistivity, specific conductance, and chloride concentration in the Biscayne Aquifer. Multiple proxy relationships are being developed using various regression methods and Archie’s Equations, and the impact of spatial variabilities in aquifer properties and groundwater composition on their accuracy is being investigated, with the ultimate goal of developing best practices for deriving and applying these proxy relationships. These best practices will be applied to improve understanding of the 3-dimensional extent of saltwater intrusion in the Model Lands, a low-lying area that hydraulically connects Everglades National Park to the Atlantic Ocean.