USING MODFLOW-2005 AND CONDUIT FLOW PROCESSES (CFP) TO UNDERSTAND THE DYNAMICS OF GROUNDWATER FLOW IN A KARST SPRINGSHED DURING FLOODS

In karst aquifers, large conduits generated by limestone dissolution focus the majority of groundwater. However eogenetic karst aquifers, such as the Floridan Aquifer, have elevated matrix porosity that can accommodate more groundwater flow than systems where primary porosity has been occluded by diagenesis and burial. Previous models that have simulated groundwater flow in eogenetic karst are concerned with regional scale flow and therefore don’t consider the interaction between the conduit and formation. This interplay becomes especially important during floods where large amounts of surface water can intrude into the aquifer via springs. During floods, heads in the conduit can become greater than heads in the formation, and the high matrix permeability can allow for significant exchange between conduit and matrix. The purpose of this study is thus to understand how conduits in the Madison Blue springshed in N. Florida affect local exchange and flow patterns in a karst system when flow reverses during flooding. Three different magnitude events were simulated using MODFLOW-2005 coupled with Conduit Flow Processes (CFP) package: a 500 year flood that saw a large scale reversal lasting several days (2009), a moderate flood that resulted in a much shorter reversal (2010), and a small event that saw minimal river water intrusion into the aquifer (2011). Each event altered flow patterns in the aquifer but to different extents. Head gradients in the conduit were a maximum of 10^-2 at and around the peak of the 2009 and 2010 events which produced maximum velocities of 0.48 m/s. The 2009 flood had twice the exchange of the 2010 flood, and four times the 2011 flood. Most of the exchange at and around the peak of the 2009 and 2010 events was flow being driven from the conduits into the surrounding formation. These results show that river reversals into conduits through spring vents as a result of flooding affect local flow dynamics. The resulting exchange of water between the matrix and conduit porosity could impact limestone dissolution and contaminant transport.