Determination of the Effect of Overwash Events On Chloride Concentrations In the Surficial Aquifers Under High Relief Dunes on Cape Lookout, Cape Lookout National Sea Grant uri icon

abstract

  • Cape Lookout National Seashore (CLNS) is a 58-mile long section on the Outer Banks, North Carolina, situated between Ocracoke Inlet and Beaufort Inlet. It includes Cape Lookout, Portsmouth Island, the Core Banks, and Shackleford Banks. The groundwater system at CLNS consists of a sandy unconfined aquifer, and two confined aquifers. On Cape Lookout, the lower confined aquifer occurs at a depth of about 340 feet, with a maximum potential yield of 500 gal/min. The upper confined aquifer occurs at depths of about 98 feet and has an estimated potential yield of 10-15 gal/min. Both the lower and upper confined aquifers are known to be fresh in the vicinity of Drum Inlet, but only the lower confined aquifer is known to contain freshwater on Cape Lookout. The freshwater potential of the confined aquifers on Cape Lookout is poorly understood and further investigation of the subsurface is necessary. Generally, low profile dunes on Cape Lookout do not exceed 5-8 feet above mean sea level (msl) and are topped by overwash from storm surges with recurrence intervals of 6 and 30 years respectively (Fig. 3) (Ho and Tracey, 1975). The residence time of saltwater in these frequently inundated lens depends on the extent of the overwash event, and the amount of post-inundation precipitation. Typically, it takes several months for a significant amount of freshwater to reclaim a surficial lens after an overwash event. Cape Lookout also has several areas where dune elevations exceed 10 feet, and it is suggested by Winner that the freshwater lenses in these areas are not affected by overwash events with recurrence interval less than 100 years (Fig. 3). These infrequently inundated lenses are protected from overwash by the dunes and may yield 20-30 gal/min of freshwater This study intends to quantify the extent of saltwater inundation on Cape Lookout to determine if the freshwater lenses underneath the high altitude dunes remain chloride free after storm derived overwash events. This information will then be used to determine the potential yield of the surficial aquifer for development. A groundwater management plan for the surficial aquifer will be constructed, outlining how to get the highest sustainable yield without inducing adverse amounts of saltwater migration from the surrounding areas. This study will also attempt to locate all wells already on Cape Lookout, and sample those wells for water quality analysis (Fig. 4). This will update the current information of the water quality in the confined aquifers under Cape Lookout.

date/time interval

  • September 2003 - January 2005