PASSIVE MONITORING TO DETERMINE TRANSMISSIVITY IN KARST AQUIFERS: AN EXAMPLE FROM O’LENO STATE PARK, FLORIDA

In karst aquifers, large-scale conduits may dominate flow, while smaller conduits, fractures, and matrix porosity provide much of the storage. To fully characterize a karst system, it is necessary to understand the hydraulic properties of each component and the coupling between them. However, determination of hydraulic properties in karst aquifers is often difficult. Aquifer tests can be used to characterize properties of the matrix/fracture systems, but can require impractically high flow rates. Transmission of natural perturbations to monitoring wells from surface water or known conduits to monitoring wells provides an alternative means to estimate hydraulic properties. Monitoring of conduits and wells at O’leno State Park, Florida during a large storm event provides an example of transmissivity determination through passive monitoring. Results were previously analyzed using a one-dimensional analytical approach, and indicated that the transmissivity values increase by > 2 orders of magnitude as the scale of measurement increases from ~100 to 4000 m. In this investigation, a cross-sectional numerical model was used to examine the effects of vertical flow due to unconfined conditions and partial penetration of the conduits in the aquifer. The results are dependent the distance of the well from the conduit, and assumptions about the active thickness of the aquifer and the conduit. For a well located 115 m from the conduit, an estimated of aquifer thickness of ~100 m, and an estimated conduit diameter of 20 m, two-dimensional simulations require a transmissivity twice that obtained from the one-dimensional analytical solution. The discrepancy becomes smaller if the aquifer is thinner than 100 m and for wells located at greater distances from the conduit.