f Use of Geophysics Methods to Characterize the Hydrology of Assateague Island National Seashore, Worcester County, Maryland

A suite of borehole and surface geophysical methods was used to characterize the lithology and salinity of the subsurface of Assateague Island, a barrier island on the Atlantic Coast of Maryland and Virginia. The U.S. Geological Survey, as part of its Climate and Land Use Change Research and Development Program, is conducting a multi-year investigation to assess potential impacts to the natural resources of Assateague Island National Seashore that may result from changes in the hydrologic system in response to projected sea-level rise. The geophysical data collected as part of the characterization phase of this investigation will serve as the foundation for development of numerical groundwater-flow models to assess the potential effects of climate change on the coastal groundwater system of Assateague Island. Geophysical data were collected along five transects across the island. Methods used included borehole electromagnetic induction and gamma, and surface time-domain electromagnetics, frequency-domain electromagnetics, ground-penetrating radar, and coupled-capacitance resistivity. Two of the transects were in areas subjected to frequent and periodic overland wash and two of the transects were in the oldest, most stable part of the island where there is no overland wash. Based on an integrated interpretation of the geophysical data, it was concluded that the geophysical methods used are effective for determining the following: approximate location of the freshwater/salt-water interface, the distribution of fresh and brackish water in the shallow aquifer, the geometry of the fresh groundwater lens beneath the island, and the distribution and thickness of fine-grained lagoonal and peat deposits in the subsurface throughout the Island.