Assessment of the Importance of Matrix Flow in a Conduit-Dominated Karst Aquifer: Investigation of the Unconfined Floridan Aquifer

University Of Florida, Gainesville FL

Investigators

Abstract

0003360 Martin In the majority of well-studied karst aquifers, the matrix rocks have low permeability and flow is dominated by conduits. Few studies have focused on karst aquifers with permeable and porous matrix rocks, although exchange of water between conduits and matrix may occur in these aquifers with important implications for contaminant storage and transportation. The unconfined Floridan Aquifer provides an excellent example of both extensive conduit development and high matrix permeability. A pilot study that focused on an ~5 km section of the Santa Fe River in north-central Florida where it flows underground through a series of conduits found qualitative evidence for exchange of water between the conduit system and matrix. The study also suggested that the extent and direction of exchange varies with the stage of the Santa Fe River. The new study proposed here will use several independent techniques to quantify the exchange and its relationship to river stage. Water level will be continuously measured at 3 critical locations along the flow path, including a contributing conduit that was recently mapped by cave diving exploration. Development of water level-discharge relationships at these locations will provide a continuous water budget for the conduit system, thereby quantifying loss or gain of water. A check on the budget will be provided by measuring changes in Cl` and SO2-4 concentrations caused by mixing of conduit water along the flow path, including a previously unsampled end-member from the contributing conduit. Chloride and SO2-4 provide the most robust measure of mixing because they are conservative and their concentrations differ greatly between the matrix and conduit water. Time-average velocities through the conduits will be measured using a thermal tracer technique developed during the pilot study and will guide water sampling. In conjunction with the fluid budget calculations, detailed time series potentiometric surface maps will be constructed, utilizing numerous karst windows, sinkholes, and a densely spaced piezometer field to be installed as part of the project. These maps will yield the head gradient between the conduits and matrix, and will allow us to estimate the matrix permeability on a field scale. The integrated results will expand current models of flow in karst aquifers by quantifying possible volumes of matrix and conduit flow, thereby refining water management practices in regions with permeable karst. The project will have extensive educational benefits through support of two graduate students, involvement of numerous student volunteers, as well as providing the focus for a newly developed course in hydrologic processes.