USING SLUG TESTS TO CHARACTERIZE HYDRAULIC CONDUCTIVITY IN A KARSTIC SINK-RISE SYSTEM

O'Leno State Park and River Rise Preserve in northern Florida provide the opportunity to study diffuse recharge to an unconfined karst aquifer, and to characterize the relationship between diffuse recharge, focused recharge from a sinking stream, and spring discharge. At this location, the Santa Fe River flows into a sinkhole and re-emerges at the River Rise, a first magnitude spring, 5 kilometers down stream. While underground, the River flows through a series of conduits and emerges at the surface at several karstic windows. The composition of the water emerging from the River Rise can range from nearly all surficial river water to nearly all groundwater from the matrix. When the River stage is high, river water can flow from the conduit into the matrix, providing a source of under-saturated water for dissolution of the matrix and a source for introducing pollutants into the aquifer. The hydraulic conductivity of the aquifer controls how water flows through it. Previous research has estimated transmissivities of the aquifer based on response of wells to head changes in the nearby conduit system; in addition, slug tests were performed on several deep wells screened at the level of the conduit (80-100 feet). In early 2006, 4 shallow wells screened at the water table (20-25 feet) were installed. In this study, slug tests were performed at these water table wells and several additional wells at the level of the conduits. Results of these slug tests provide a picture of the hydraulic conductivity near the conduit, far from the conduit, and at the water table. Significant variation in hydraulic conductivities was found. The hydraulic conductivity for three of the shallow well sites was 2-3 times larger than the collocated deep wells. For most of the deep wells, the hydraulic conductivity results from the slug tests are consistent with transmissivity results calculated for the same wells from recession curves following storm events. In contrast, for two of the deep wells transmissivities calculated from slug tests are significantly higher than calculated from recession curves, and are more consistent with results of transmissivity estimates based on one-dimensional modeling of aquifer response to storm events.