Effects of Antecedent Hydrogeologic Conditions on Flood Magnitude and Recharge to the Floridan Aquifer in North- Central Florida

Water is one of our most important resources. Both surface water and groundwater are used by humans for drinking, irrigation, transportation, entertainment, and are a major service for ecosystems. Almost all surface water features, such as streams, rivers, and lakes, interact with groundwater by exchanging water between these two reservoirs. Surface water can become polluted depending on land use. It may also exchange with groundwater and cause degradation of groundwater quality. Contaminated groundwater can also degrade pristine surface water when it returns to the surface, for example at springs. Thus, an understanding of the interactions between groundwater and surface water is required for effective water management. One of the most productive aquifers in the world is the Floridan Aquifer, a large karst aquifer. Karst aquifers have high permeability over large scales, which allows for fast exchange between surface water and groundwater (Kincaid et al., 1997). Many karst aquifers form in extensively cemented and recrystallized Mesozoic and Paleozoic carbonate rocks (White, 1969; Smart et al., 1986; Ford et al., 1989). The Floridan Aquifer formed in Cenozoic rocks and thus retains much of its primary porosity, a characteristic referred to as eogenetic (Vacher and Mylroie, 2002). Flow through the primary porosity of the matrix rocks within eogenetic aquifers can be orders of magnitudes faster than within the low porosity matrix rocks of older aquifers (Vacher and Mylroie, 2002). Contamination may be more common in eogenetic karst systems, like the Floridan Aquifer, because of extensive exchange between surface water and groundwater. The processes controlling exchange between surface water and groundwater are not fully understood (Winter et al., 1998). One possible control on the magnitude of the exchange between surface water and groundwater, and associated influx of contaminants to the groundwater, is the elevation of groundwater relative to surface water. Elevations of both surface water and groundwater may change rapidly or over long periods of time depending on rainfall, flooding, and droughts, and these two elevations may not change in concert with each other. Consequently, we assess how exchange of surface water and groundwater is controlled by variations in water levels in the Floridan Aquifer and estimate the magnitude of recharge of water to the Floridan Aquifer during flooding.