Intellectual Merit. It has been recognized recently that the flow field of karst aquifers includes conduits, fractures, and intergranular porosity of the matrix and that full characterization of flow requires understanding flow in each component, coupling between the components, and how water recharges to the aquifer from the surface. Most well-studied karst aquifers have low intergranular porosity where diffuse flow is confined to a network of fine fractures surrounding conduits. In contrast, this proposed work centers on the Floridan aquifer, which retains high (up to ~20%) intergranular matrix porosity. Previous work on physical hydrogeology (discharge measurements, temperature tracing of flow rates, observations of hydraulic head), suggests that water exchanged between conduits and the matrix of the Floridan aquifer is controlled by reversals in head gradients between the matrix and conduits as a result of high allogenic input into the conduit system. The previous work has not quantified contributions from diffuse recharge from the surface, the magnitude of exchange between the conduits and matrix, the residence time of water that exchanges, or chemical modifications of the aquifer through dissolution reactions. Consequently, this proposed project will determine coupling between aquifer components and water sources by integrating new chemical measurements with previously used physical observational techniques. The field area will be the Santa Fe Sink/Rise system in north-central Florida. Observations to be made will include major element and isotope compositions of water from nested new and existing wells and lysimeters (soil water samplers) for ground water characterization, from the River Sink, Rise, and karst windows for characterization of conduit water, and from precipitation for characterization of the input sources. The chemical measurements will provide flow velocities to be compared with head gradients between the conduits and the matrix and saturation states with respect to carbonate and silicate minerals. The nested wells and data on chemical composition of precipitation will allow characterization of vertical differences in water chemistry to determine magnitudes of diffuse recharge from the surface. These data will be compared to other karst aquifers with low intergranular porosity using available deterministic models such as chemograph separation. Comparisons will also be made to results of previous and ongoing studies of other regions of the Floridan aquifer. Because of the higher transmissivity of the highly porous Floridan aquifer, it is anticipated that the intergranular porosity component will play a large role in the overall flow field of the aquifer. Broader Impacts. This work should improve the understanding and management of karst aquifers that have significant portions of their flow within matrix porosity. These aquifers commonly represent major water resources, but have previously been managed using conceptual models that consider either flow primarily through conduits or treat the system as an equivalent porous medium. In particular, these results will be directly applicable to management of the Floridan aquifer and results will be disseminated to critical water managers through Martin's participation in the Florida Springs Task Force. Human resources will be impacted through graduate and undergraduate education, both formally as a resource for currently taught courses and through graduate student theses. One second-year PhD student is currently working on this topic as part of his dissertation, and two masters students and several undergraduates will participate in the field, laboratory, and analytical work.
