Using Numerical Modeling, Age Dating, and Geochemical Analyses to Delineate Contributing Areas to Public Supply Wells in the Karstic Upper Floridan Aquifer, Tallahassee, Florida

Ground water from the karstic Upper Floridan aquifer (UFA) is the sole source of water supply for Tallahassee, Florida, and surrounding areas of Leon County. The City of Tallahassee (City) currently operates 28 water-supply wells. Most of these wells yield an ample supply of potable water; however, low levels of tetrachloroethylene (PCE) have been found in water from several wells. The City removes the PCE from the water by passing it through granular-activated carbon units before distribution. To ensure that water-supply wells presently free of contamination remain PCE-free it is necessary to understand the ground-water flow system in sufficient detail to protect the contributing areas. The City cooperated with the U.S. Geological Survey (USGS) in developing a ground-water flow model of the UFA to delineate contributing areas of water-supply wells and conduct age dating. Ground-water samples collected from four public-supply wells along a north-south transect were analyzed for tritium, helium-3, chlorofluorocarbons (CFCs), and sulfur hexafluoride (SF₆). All three dating methods indicate that the apparent age of ground water generally decreases from northern to southern Leon County. This southward trend of decreasing ages is consistent with increasing amounts of recharge that occur as ground water moves from north to south. Ground-water age data derived by geochemical analyses were used in combination with ground-water flow modeling and particle tracking to determine an effective porosity for the Hawthorn clays and UFA. The Hawthorn clays range from a few meters thick to hundreds of meters thick and extend from land surface (or near land surface) down to the top of the UFA. The effective porosities for the UFA that resulted in best model matches were averaged to determine an average effective porosity of 7 percent, and the effective porosities for the Hawthorn clays that resulted in matches were also averaged to produce an effective porosity of 22 percent. Probabilistic contributing areas were determined for 26 wells using MODFLOW and MODPATH. Five-year time-dependent capture zones assuming effective porosities of 0.1, 1, and 7 percent were delineated for four representative wells. Higher probabilities of capture (greater than 40, 60 and 80 percent) of the 5-year time-dependent capture zones were similar for all effective porosities; however, the lower probabilities of capture showed a large range of variability. The combination of ground-water flow modeling, age dating, and geochemical analyses provided critical information for a better understanding concerning recharge areas to public supply wells in this karstic aquifer system.