HYDROGEOLOGY OF PONDS AND GROUND WATER ON ASSATEAGUE ISLAND, MARYLAND

A representative set of eleven ponds on Assateague Island, a barrier island and National Seashore on the Atlantic coast of Maryland, was characterized and monitored weekly for one year to evaluate the interaction between pond surface water and ground water. The physical setting of each pond was interpreted for geomorphic context, mode of formation, and associated soils and plant communities. DEMs of each site were developed from LIDAR data and interpreted along with a high-resolution aerial photomosaic and historical aerial photographs. Weekly monitoring of all ponds included measurements of water level, temperature, conductivity (salinity), and pH, and was supplemented with continuous records of water level in the ponds and of the adjacent water table at four focus sites. Pond water levels and conductivities were related to precipitation and potential ET from weather data collected on the island, and to elevated water levels and wave heights in the ocean that may have produced seawater overwash. The distribution of fresh and saline ground water, and the depth of the freshwater lens were measured at the focus sites by surveys with electrical resistivity and ground-penetrating radar. Some geophysical transects were reoccupied to document temporal changes associated with response and recovery from overwash events. Increased salinities of pond water were attributed to four primary processes: (1) direct surface overwash of seawater into the pond; (2) discharge of saline ground water into the pond from upgradient overwash and infiltration; (3) direct flooding from the bay side; and (4) input and subsequent concentration by evaporation of salt spray from the ocean (this was a minor effect). The hydrogeologic setting of each pond was linked closely to storm processes that shaped the surface morphology of the island. Ponds most commonly occupied deeply scoured overwash channels, swales between storm ridges, or near the center of “wash-arounds.”