HYDROGRAPHS FROM THE 2004 HURRICANE SEASON IN THE EOGENETIC KARST OF THE FLORIDAN AQUIFER

The eogenetic karst of the unconfined Floridan aquifer differs from telogenetic karst in the way it responds to rainfall. Hydrographs vary according to seasonal or longer-period cycles because of the large storage provided by the highly conductive matrix. Discharge from Florida springs rarely show single storm events. The 2004 hurricane season provided a unique opportunity to investigate storm events large enough to overcome the inertial effects of large storage within the aquifer. Water-level observations using piezometers from nine locations in three portions of the unconfined Floridan aquifer show a variety of responses to Hurricanes Frances and Jeanne, which came twenty days apart. At Morris Cave in the Radar Hill quarry near Brooksville, storm short-duration and high-amplitude peaks overprint a background trend reflecting seasonal change in water level. The spiky storm hydrographs, similar to those from telogenetic karst aquifers, are driven by artificial recharge through a cave entrance created by quarrying. Nevertheless, the background trend is not affected by the short-term spikes thus indicating the huge storage in the aquifer. In Briar Cave near Ocala, where passages follow recognizable fractures, water levels change seasonally in sync with the background trend at Morris Cave. Individual storm events below a threshold level have no effect on the water level. Rainfall from the two hurricanes exceeded this threshold causing a rapid increase in the water level at the cave and in the discharge at nearby Rainbow Springs. The increase in measured drip rates from matrix inside Briar Cave lags behind the rise in the water level in the cave suggesting that fractures control the water-level rises as well as the directional morphology of the caves in the Ocala area. Seven piezometers near High Springs, northwest of Gainesville, display a range of responses to the hurricanes. One well close to the Santa Fe River penetrates a large cavity. The rise and fall of water in this well is consistent with a conduit connection to the river. Data from three sites further from the river are similar to Briar Cave, and three sites even further from the river have a signal similar to Morris Cave, suggesting, by analogy, that fractures and then matrix, respectively, may play increasingly more dominant roles away from the river.