Nuclear Magnetic Resonance Imaging of Groundwater Flow within the Matrix Porosity of the Biscayne Aquifer of Southeast Florida

In this study, we use an innovative, non-invasive technology, nuclear magnetic resonance imaging (NMRI), to visualize the direction and magnitude of ground water flow in field samples of late Pleistocene limestone of the Biscayne aquifer. Specific goals of the first set of NMRI experiments are to map the advective velocity of water flowing at two rates of specific discharge (0.00025 and 0.00013 m/s) through a 10-cm diameter cylindrical, epoxy-resin model. The model interior accurately reproduces a well-connected maze of ichnologically influenced, centimeter-scale, touching-vug macroporosity common within preferred flow zones in parts of the Biscayne aquifer. A second set of NMRI experiments investigates the migration of freshwater into the matrix of permeable (gas minipermeameter mean 10^-13.5 m² on four samples) and porous (mean of 44%, on four samples) peloid-oöid grainstone initially saturated with heavy-water (D₂O). In the experiments on the model, we generate velocity maps using phase-encoded, stimulated-echo imaging. In the experiments on the rock matrix, the focus of this abstract, we visualize the progressive replacement of D₂O in the rock matrix using sequential time-step images of NMRI signal strength.

Results for the freshwater-D₂O experiments reveal a substantial flux of freshwater into the matrix porosity with a simultaneous loss of D₂O. Specifically, we measured rates upward of 0.001 milliliters per hour per gram of sample (mL/hr-g) in static or non-flowing conditions, and perhaps as great as 0.07 mL/hr-g when freshwater continuously flows past a sample at velocities less than those found within stressed areas of the Biscayne aquifer. These experiments illustrate how freshwater and D₂O, with different chemical properties, migrate within one type of matrix porosity found in the Biscayne aquifer. Furthermore, these experiments are a comparative exercise in the replacement of seawater by freshwater in the matrix of a coastal, karst aquifer, since D₂O has a greater density than freshwater.