Stratigraphically Controlled Freshwater Lens Development in a Barrier Island, Padre Island, Texas

Barrier island groundwater systems are typically comprised of an unconfined, precipitation-derived, freshwater lens that overlies denser saltwater from the adjacent ocean and lagoon. Accurate characterization of groundwater flow dynamics and morphological description of the freshwater lens is increasingly important as anthropogenic changes to such systems become pervasive. We are currently investigating the dynamics and morphology of the freshwater lens along a cross-island transect at South Bird Island Basin, Padre Island, Texas. Along transect, water table elevations, salinity, and freshwater/saltwater interface depths were measured via temporary monitoring wells, refractometer, and controlled-source magnetotelluric surveys (CSMT) respectively. A stratigraphic profile of the upper 10-15 meters is being constructed using 50 MHz ground penetrating radar (GPR) data.

The elevations of the water table and the freshwater/saltwater interface bound a freshwater lens that deviates significantly from analytical solutions and homogeneous finite-difference models in both symmetry and overall lens thickness. (1) The freshwater lens thickness is significantly less than that predicted by the models, and is variable under the central island. (2) The lens on the Laguna Madre side of the island has a significantly attenuated thickness up to 500 meters from the shore. The first observation is likely explained by anisotropic aquifer properties, and subjacent, low-permeability strata associated with the heterolithic Beaumont Formation at a depth of 10-20 meters. The second observation is consistent with GPR interpretations of island stratigraphy that suggest inter-bedded washover fans and low-permeability Laguna Madre muds are present on the landward side of the island.

A two-dimensional variable-density groundwater flow model (SEAWAT) incorporating such stratigraphic complexities is being formulated and calibrated to freshwater lens observations. These modeling efforts should help quantify the freshwater lens dynamics so that scenarios incorporating anthropogenic disturbances can be addressed.