GEOMORPHOLOGY AND THE DISTRIBUTION OF FRESH GROUND WATER ON ASSATEAGUE ISLAND, MARYLAND

Fresh water is a valuable natural resource on a narrow, sandy barrier island such as Assateague Island on the Atlantic coast of Maryland. The availability of fresh water is fundamental for maintaining the terrestrial plant and animal communities on the island. This study has evaluated the occurrence, distribution, and temporal variation of fresh water in ponds and ground water on Assateague Island. The pond component of the study monitored weekly for one year a set of representative ponds ranging in size from 5 to 1000 m² and ranging in salinity from perennially fresh to frequently inundated with full-salinity seawater. Physical characteristics, water-quality parameters, typical soils, and hydrogeologic setting were documented for each pond, and mode of formation was inferred from historic aerial photographs. The interaction of the ponds with ground water in the surficial aquifer of the island was assessed with staff gauges and water-level recorders installed in the ponds and recorders on nearby water-table wells. Geophysical surveys were conducted at each pond site, using ground-penetrating radar and electrical resistivity to map subsurface stratigraphy related to permeability and ground-water flow, and to define the depth to saline ground water underlying the fresh-water lens near the surface. Several sites that received salt-water overwash during moderate coastal storms during the study period were surveyed soon after the storm and several months later to measure the flushing of salt water through the aquifer. The microtopography (geomorphology) of the island surface created during major storms of record in part controls the frequency and intensity of salt-water overwash and inundation on both the seaward and bayward sides of the island. This relationship is also reflected in the distribution of vegetation communities on the island.