Hydrologic, Geophysical, and Geochemical Characterization of an Aquifer along the Beach of a Barrier Island
Date:
Abstract:
Coastal aquifers are of global importance. They nurture marine ecosystems and support billions of people living near the coast. Coastal groundwater resources are particularly important for small island communities like Mustang Island, Texas, where rising sea levels, violent storm surges, and urbanization seriously threaten the island’s aquifer. Mustang Island is a barrier island formed by sediment deposition during the last ice age. The permeable island foundation supports a small freshwater aquifer that perches atop saltwater, i.e., a freshwater lens. Freshwater lenses rely on rainfall for recharge and are susceptible to changes in sea level, including from storm surges. Consequently, freshwater lens aquifer systems frequently experience significant fluctuations in shape and extent. Here, as part of a field methods class at the University of Texas at Austin, we report the results of geophysical (electrical resistivity [ER]), geochemical, and hydraulic observations along a beach-perpendicular study transect at Port Aransas beach on Mustang Island. We mapped a water table inclined towards shore, and that changed with the tide. Our observations suggest the water table and unconfined aquifer were responding to a storm surge which occurred immediately prior to our field study. Groundwater salinity (and water electrical conductivity) increased toward the shoreline. ER imaging showed distinct zonation within the water table, measuring groundwater resistivity ranging from 2.0 - 3.6 Ohm-m between 1.5 to 3 m below the surface and groundwater resistivity of 1.1 - 1.7 Ohm-m within 1.5 m of the surface and below 3 m. Measurements of aquifer hydraulic conductivity (K) displayed distinct spatial heterogeneity, with the highest K-values measured near the shore, dunes, and 15 cm beneath the surface of the beach. The analyses of ER, geochemical, and K-value data were used to generate geochemical and geophysical models of the groundwater to better understand the evolution of the freshwater lens in the presence of a dynamic, saline tide.