TOPOGRAPHIC CONTROL ON BARRIER ISLAND FRESHWATER LENSES, MUSTANG ISLAND, TEXAS

Barrier islands host one of the most sensitive ecosystems in the world because of the permeability of the sandy substrate, the mobility of poorly consolidated sands, and the pervasive salt spray. Thus, they are particularly vulnerable to changes in sea-level, climate, and land use. Of particular concern is the amount of freshwater in barrier islands in semi-arid regions of South Texas which is often the only source of water for sand-anchoring plants during extended dry periods. Barrier island aquifers are unconfined, precipitation derived, freshwater lenses that overlie denser saltwater from the adjacent ocean and lagoon. Characterization freshwater lenses is increasingly important as land use intensifies and economic development progresses.

This purpose of this study is to examine how the topography of coastal dune fields controls the extent of the freshwater lens. Preliminary data suggests that large coastal dunes increase the extent of subsurface freshwater. This is likely due to enhanced recharge through dry dune sand, closed basins of the hummocky topography, and reduced water availability to plants because of the deep water table. In contrast, dune fields that have been replaced by blowouts, lowlands, and wetlands likely result in enhanced evapotranspiration and evaporation from flooded depressions after rainfall.

This relationship is being evaluated in Mustang Island State Park where a topographic contrast exists between a zone with extensive dune development, and a zone that has been degraded leaving only a small foredune ridge and back-island wetlands. The distribution of freshwater is being investigated using a variety of techniques. The conductivity of the beach was mapped from the tide line to the dunes using an EM-31 conductivity survey, and vibracores were recovered from the backshore. Finally, controlled-source magnetotelluric surveys (CSMT) were conducted on two shore perpendicular transects to generate soundings of electrical conductivity. Results show that recent activity from hurricane Ike had inundated some of the shore sands with saltwater but the freshwater lens was still present under the dunes. In addition, the freshwater lens below the zone with large dunes was more extensive than the lens below the zone with degraded dunes.