MATRIX HETEROGENEITY IN AN EOGENETIC KARST AQUIFER AND IMPLICATIONS FOR PREFERENTIAL FLOW PATHS

Understanding preferential groundwater flow paths is difficult in karst aquifer systems where both matrix porosity and conduits contribute to flow. O’Leno State Park in north-central Florida is an ideal site for studying groundwater flow paths in a high matrix porosity (i.e., eogenetic) karst aquifer. Here, the Santa Fe River flows into a 36-meter-deep sinkhole and emerges 5 km away at a first magnitude spring. While underground, the river flows through a series of limestone conduits, but during flood conditions river water is recharged into the surrounding aquifer matrix due to elevated hydraulic head in the conduits. Changes in water level, specific conductivity, and temperature in the matrix groundwater during flooding were monitored by four pairs of nested wells screened at the water table and at the level of the conduit. Changes in water level caused by flooding from a tropical storm in 2008 were detected in all wells. In contrast, shifts in specific conductivity were only observed in shallow wells during and following the flood. Slug tests in the water table and conduit level wells suggest hydraulic conductivities are approximately an order of magnitude higher at the water table than at the depth of the conduit. This elevated hydraulic conductivity may result from dissolution from diffuse recharge of undersaturated water reaching the top of the limestone through the overlying siliciclastic sands. Numerical modeling suggests that the difference in hydraulic conductivity between the water table and at conduit depths is enough to drive preferential flow from the conduit to the water table. These results suggest that dissolution at the water table also occurs from allogenic recharge flowing from the conduits to the water table, resulting in a feedback effect of dissolution caused by diffuse and allogenic recharge.