USE OF SPECIFIC CONDUCTANCE IN MONITORING SALTWATER INTRUSION IN THE BISCAYNE AQUIFER, SOUTHEAST FLORIDA

The Biscayne aquifer in southeastern Florida is a transmissive, heterogeneous limestone aquifer that is affected by saltwater intrusion. Chloride concentrations are a common indicator of saltwater intrusion in coastal aquifers. Chloride data from wells located close to the freshwater-saltwater interface and open near the base of the aquifer, where saltwater is expected to occur, are needed to determine the inland extent of saltwater intrusion. The position of the freshwater-saltwater interface is transient, and wells may lose their relevance for saltwater intrusion monitoring as the salt front migrates inland. Thus, accurate identification of the inland extent of saltwater intrusion is limited by the availability of wells that are suitably located and constructed. Groundwater chloride-concentrations are typically obtained from laboratory analysis of a groundwater sample, whereas specific conductance can be measured in the field using a portable water quality probe and handheld meter. Therefore, many wells that are not monitored for chloride concentration are monitored for specific conductance, which can be used as a proxy for chloride concentration. To increase the coverage of saltwater-intrusion monitoring, regression models are often used to estimate chloride concentration from specific conductance. Specific conductance is a bulk value that results from the conductances of all the ions in a solution. Therefore, the relationship between chloride and specific conductance in groundwater varies with the overall chemical composition of the water. At least eight unique regression models for predicting chloride from specific conductance measurements have been documented for the Biscayne aquifer. However, a rigorous analysis that compares various regression models, includes residual analysis, and considers spatial variability and model performance over a range in concentrations is lacking. This presentation will summarize an in-depth investigation of a dataset of ~1900 samples, with both chloride and specific-conductance measurements, from 170 wells that was compiled from various monitoring programs in Miami-Dade County from 1995 to 2023. Thorough evaluation of this data set will facilitate development of best practices for using specific conductance as a proxy for chloride concentration when monitoring saltwater intrusion in the Biscayne aquifer.