Groundwater surface water mixing in a semi-confined karst aquifer using field and particle tracking forward-model inversions

In the upper Santa Fe River Basin, surface water flows into karst conduits of the groundwater system. Surface water may then recharge and exchange with groundwater in porous matrix of the Floridan aquifer, depending on differences in the head in the conduits versus head in the matrix, which is controlled by discharge at the Santa Fe River sink-rise system. However, the spatial extent of mixing and exchange dynamics at different flow conditions is unknown. In our study, we directly examine the mixing of karst conduit and matrix waters and how spatial and temporal patterns of mixing change during high flow and baseflow conditions. Using an electrical resistivity tomography dataset, we quantify spatial mixing based on time-lapse electrical conductivities at two separate field sites. We then simulate flow gradients based on head between karst conduits and matrix to determine fractions of groundwater and surface water in matrix and conduit during high flow and baseflow using our particle-tracking model. These particles represent groundwater-surface water concentrations, which are used as a representation of resistivity. Using field and forward model time-lapse inversions we directly compare observations that reflect exchange dynamics, spatial mixing and flow conditions. This comparison provides details on matrix-conduit mixing and temporal dynamics at different flow conditions, illustrating the spatial extents of recharge to the porous matrix of a karst groundwater system.