OCCURRENCE OF BRACKISH AND FRESH GROUND WATER BENEATH MID-ATLANTIC BARRIER ISLANDS AND ESTUARIES

As part of ongoing coring and geophysical studies of Atlantic coastal aquifers, recent investigations have examined the salinity and hydrostratigraphy of ground water beneath barrier islands and adjacent marshes and estuaries in North Carolina, Maryland, and Delaware. Several general patterns have emerged from these efforts, with important implications for both water supply and coastal ecosystem health issues.

Surficial fresh-water lenses under barrier islands tend to vary in thickness, with maxima existing under island segments with either the greatest width or the most topographic relief. The cross-island profile of fresh lenses indicates general asymmetry, with greater thickness on the seaward side. This assymetry may result from either dune-influenced recharge (NC), or from the influence of confining units deposited in back-barrier settings and incorporated into island subsurface zones as barriers migrate landward (MD). Fully marine salinity of ground water immediately below the fresh surficial lenses on many barrier islands indicates possible recharge of saline water from the exposed beachfront and submarine units, rather than infiltration and underflow of brackish estuarine water from behind the islands.

The thickness of the saline-brackish zone beneath the surficial lenses generally increases with increasing width of the back-barrier lagoon. This is clearly shown in a series of borings on the Outer Banks of North Carolina, adjacent to Pamlico Sound, although the influence of local variations in lithostratigraphy is also important here. Subestuarine transport of fresh to slightly brackish water recharged on the mainland appears to be common in the clastic-dominated stratigraphic settings studied, which has also been observed in various karstic systems in Florida. Plumes or layers of fresh to slightly brackish water derived from surficial or confined aquifers on land travel hundreds of meters or more beneath estuaries, where conduits and confining units are present. In some cases, such water passes completely under both lagoons and barriers, eventually discharging to the open ocean. Deep fresh water such as this has been intercepted for many years for drinking water on some barrier islands (MD). Slightly brackish examples of these waters have been used more recently as feedstock for reverse osmosis plants (NC, FL, NJ proposed).