CONTROLS OF SUBSURFACE GEOLOGY AND SURFACE GEOMORPHOLOGY ON BARRIER ISLAND’S FRESHWATER LENS: INFERENCES FROM GEOPHYSICAL AND TOPOGRAPHICAL SURVEYS

Coastal regions, home to approximately 38% of the global population, face unique challenges in managing freshwater resources. Barrier islands, valuable for their ecological, economic, and protective functions, rely on precipitation-recharged surficial and shallow groundwater aquifers. This freshwater, critical for both human and ecological communities, forms a lens atop denser saltwater. The extent and availability of this freshwater lens (FWL) is influenced by subsurface geology, recharge rates, surface topography, vegetation cover, and development density.

This study investigates the role of two key factors in controlling the geometry of the FWL on barrier islands: subsurface geology, specifically the depth to low-permeability layers, and surface elevation. We employed high-resolution time-domain electromagnetic (TDEM) and digital elevation model (DEM) (1 m) data to examine these relationships on Padre Island, Texas, the world's longest barrier island. A total of 38 TDEM soundings were collected along three transects extending across the island, from the Gulf of Mexico to the hypersaline Laguna Madre, capturing variations in FWL, subsurface geology (e.g., the Pleistocene clay layer), and surface topography.

The measured elevations ranged from 0.6 to 3.0 m above sea level over the three transects. The apparent resistivities of the FWL ranged from 8.5 to 82.3 Ω.m. The maximum depth to the Pleistocene clay layer was found to be around 20 m, dipping mainly towards the Gulf of Mexico. Notably, the FWL was consistently absent in areas with elevations at or below 1.2 meters above sea level. This absence was observed not only in areas near the Laguna Madre and Gulf of Mexico but also unexpectedly in some central island locations. The thickness of the FWL correlated with both island elevation (r = 0.57) and depth to the impermeable layer (r = 0.64). These results suggest that both surface elevation and subsurface geology play significant roles in controlling freshwater availability on Padre Island, highlighting the importance of considering both factors in barrier island water management strategies.