SALINE AND HYPERSALINE GROUNDWATER UNDER ASSATEAGUE ISLAND, MARYLAND

As part of a larger study of the groundwater system under Assateague Island, Maryland, we have investigated the distribution of a hypersaline brine beneath a small section of island. This brine was first identified from groundwater samples taken from a U.S. Geological Survey well cluster, which have salinities as high as 77 ‰, or more than twice seawater salinity. Subsequent electromagnetic-induction and gamma logs of the wells showed that brine extends from 7 to 17 meters below the surface, and is in a thick sand body interpreted as a tidal-inlet channel. The sand and the brine are underlain by a silty confining layer. Ground-penetrating radar and resistivity surveys were conducted to map the subsurface distribution of the brine under Assateague Island. During the summer, evaporation of seawater in back-barrier salt marshes produces the brine. Because of its high density, the brine sinks through the groundwater, possibly by salt fingering, and flows along the silt confining layer to pool in the old inlet channel. We have created a conceptual model that explains how the brine formed and how it affects groundwater flow under the barrier island. This model can be applied to other barrier-island systems, and may explain the sporadic occurrence of brines.