Description: Maps of groundwater recharge areas are useful planning tools for groundwater resource management. Sections 373.0391, 373.0395, and 373.0397, Florida Statutes, require the water management districts to provide recharge area information to local governments and planning agencies. Recharge areas of the Floridan aquifer in the St. Johns River Water Management District (SJRWMD) were mapped using a geographic information system to analyze the geologic and hydrologic factors that affect recharge.The hydrogeologic characteristics of the surficial aquifer system, the intermediate aquifer system and confining unit, and the Floridan aquifer system determine areas and rates of recharge to the Floridan aquifer in SJRWMD. The surficial aquifer system extends from the land surface downward to the top of the intermediate confining unit separating the surficial and Floridan aquifer systems. The water table of the surficial aquifer is the upper surface of the zone of saturation. The intermediate confining unit, where present, retards the movement of water between the surficial and Floridan aquifers and confines the Floridan aquifer under artesian conditions. The water level in a well tapping the Floridan aquifer is called the potentiometric level, and this water level can be higher, lower, or at the same elevation as the water table. Leakance of water through the confining unit is dependent on its thickness and hydraulic conductivity. Recharge to the Floridan aquifer occurs in areas where the water table elevation is higher than the Upper Floridan potentiometric elevation, creating downward hydraulic pressure to move water through the confining unit. In areas where the Upper Floridan potentiometric surface is at a higher elevation than the water table, leakance is upward from the Floridan to the surficial aquifer. The areas of upward leakance define the discharge areas of the Floridan aquifer. Where Floridan potentiometric elevations are above the land surface, springs and free-flowing artesian wells occur. Recharge rates to the Floridan aquifer were calculated based on an analysis of the hydraulic pressure differences between the surficial aquifer water table and the Floridan potentiometric surface, and on the leakance of water through the confining unit separating the aquifers. Recharge also occurs directly from infiltrating rainfall where the limestones of the Floridan aquifer are at or near land surface. Significant recharge may also occur where the confining unit is breached by sinkholes and sinkhole-related features, depending on the hydrogeologic relationships between the surficial and Floridan aquifers. Recharge rates were mapped at a 4 inch/year contour interval. The following mathematical operations were performed to determine confining unit leakance, hydraulic pressure differences between the surficial and Floridan aquifers, and recharge to the Floridan aquifer: Hydraulic pressure difference = water table elevation minus Floridan potentiometric surface elevation Leakance = vertical hydraulic conductivity of confining unit divided by thickness of confining unit Recharge = hydraulic pressure difference multiplied by leakance The input raw data used for the hydraulic pressure difference map (water table elevation minus Floridan potentiometric elevation) and the confining unit thickness map (confining unit elevation minus Floridan aquifer system elevation) were compiled from SJRWMD databases. While a large number of data points were available for these data layers, the data are concentrated in some areas and sparse in other areas. Geostatistical methods using the ISATIS software program were used to analyze the spatial characteristics of the data to produce the best estimation of each data layer throughout SJRWMD. Grid outputs (115 meter x 115 meter) for each layer were exported to ArcMap to perform the mathematical operations described above. Heterogeneity in confining unit properties and lack of vertical hydraulic conductivity data from cores made it impractical to estimate conductivity using geostatistics. As a result, sensitivity analysis on available conductivity data was used to determine the conductivity value and leakance range that most closely approximated leakance values derived from groundwater flow model calibrations. This December 2004 Floridan aquifer recharge map updates the map published in Technical Publication SJ93-5, "Recharge Areas of the Floridan Aquifer in the St. Johns River Water Management District." The recharge map provides a regional assessment of recharge to the Floridan aquifer. The map is intended to be used as a regional planning aid for groundwater resource management. The map is not intended for site-specific assessments.