WHAT IS THE INTERACTION BETWEEN CONDUIT AND MATRIX POROSITY? AN EXAMPLE FROM THE SANTA FE RIVER SINK/RISE SYSTEM

In carbonate aquifers three elements of porosity can occur in the form of conduits, fractures, and matrix. In uncrystallized systems matrix permeability may allow water to exchange between the matrix and conduit porosity. Here we use major element chemistry, Sr concentration, and Sr isotope ratios to determine this exchange at the Santa Fe River Sink/Rise system in north-central Florida, along the boundary of the confined and unconfined Floridan Aquifer. Between the River sink and Rise numerous karst windows result from collapse of conduits. Water samples from the Sink, Rise, 12 karst windows, and 6 wells that were collected between May 2002 and April 2003 indicate that river stage plays an important role in conduit/matrix exchange. During low stage, the karst windows have higher cation concentrations than during intermediate and flood stage. Sr concentrations, for example, average 2ppm, 0.19ppm, and 0.03ppm during low, intermediate, and flood stage, respectively. Plots of ⁸⁷Sr/⁸⁶Sr versus 1/Sr concentrations reflect two end member mixing, most likely surface water with its Sr chemistry (Sr=0.025ppm, ⁸⁷Sr/⁸⁶Sr=0.709481) modified by reactions with clay minerals in the confining unit and water from the matrix porosity (Sr=1.6ppm, ⁸⁷Sr/⁸⁶Sr=0.707843) with some Sr originating from the Eocene and Oligocene calcite of the Floridan Aquifer. A two-end member mixing model using Sr concentrations and isotope ratios shows that samples collected from the karst windows at low stage lie near the matrix water end-member, samples from flood stage lie near the surface water end-member, and samples collected during intermediate stages lie between the two, reflecting a mixture of both surface water and matrix water. Samples from the wells, particularly those closest to the conduit, have slightly elevated ⁸⁷Sr/⁸⁶Sr ratios during both intermediate and flood stages, reflecting infiltration of surface water to the matrix porosity.