A REGIONAL APPROACH TO ASSESSING GROUNDWATER AVAILABILITY OF THE FLORIDAN AQUIFER SYSTEM

The Floridan Aquifer system (FAS) is one of seven principal aquifers in the United States that is currently being evaluated as part of a national assessment of water availability by the U.S. Geological Survey Groundwater Resource Program. The objectives of the FAS study are to quantify current groundwater resources, evaluate how these resources have changed over time, and provide tools to forecast system responses to stresses from future human and environmental uses.

Assessing the groundwater availiabilty of the FAS poses many challenges because of the large geographic size of the area (approximately 100,000 square miles underlying all of Florida, and parts of Georgia, Alabama, South Carolina, and Mississippi), and the complexity of groundwater/surface water interactions. The FAS is a karst system with varying lithologic rock units that correspondingly have widely varying hydrologic properties, such as hydraulic conductivity. The FAS is confined in some areas, but with some unconfined areas containing numerous springs supplying water to lakes and streams. It is hypothesized that groundwater withdrawals for agriculture, municipal, and industrial use could be decreasing spring discharge from the FAS. A large-scale numerical groundwater flow model is being developed to aid in assessment of the overall availability of water by estimating historical conditions and predicting future conditions, such as spring and stream discharge.

Another challenge in model development is representing the fresh water-saltwater interface. Runtimes for a full variable-density flow and transport model of the entire Floridan aquifer could be prohibitively long. However, neglecting salinity may result in overestimating the amount of fresh water. A compromise that would allow estimation of the fresh water/saltwater interface is a simulation using a sharp-interface approach. For example, the Sea Water Interface (SWI) Package for MODFLOW2000 offers relatively short runtimes, while simulating dynamic interface flow. Utilization of the SWI package is currently being tested for the FAS because the sharp-interface approach would allow an assessment of groundwater availability that considers the reduction in available fresh groundwater in coastal areas as a result of groundwater pumpage, climate change, and sea-level rise.