Cape Hatteras

National Seashore
North Carolina

U.S. Department of the Interior
National Park Service

    The North Carolina barrier islands lie in a world of water. They are bounded on the east by the vast Atlantic Ocean. To the west, shallow sounds up to thirty miles wide lap their shores. The water resource seems endless. The picture changes when focus shifts from salt water to fresh. Suddenly the resource becomes limited and precious. Only through a delicate balance are these islands able to store pockets of fresh water under their sandy soils.

    With each rainfall, water disappears into the porous soil. Some flows toward the edges of the island and seeps into the ocean and sounds. Remaining rainwater filters downward where it meets underlying sediments saturated with salty ocean water. Some intermixing occurs. However, being less dense, much of the fresh water forms a "floating" lens above the saltwater layer. Thus a ground water aquifer is formed. The fluid boundary between the fresh and salt layers fluctuates. Tidal changes affect the position of the salt zone. Variations in fresh water input can cause the aquifer to shrink or expand. Additional rains are essential for maintaining and recharging the aquifer.

    The Outer Banks which comprise Cape Hatteras National Seashore and its surrounding communities are low and narrow. The land's capacity to store water is not great. On average, the aquifer is no more than 15 feet thick. In Buxton Woods, where Hatteras Island is relatively wide and high, the aquifer is well developed. The land stretches three miles across. Long, parallel rows of wooded ridges up to fifty feet high divide the landscape.

    Between the ridges of Buxton Woods, the lowlands dip below the surface of the ground water, creating fresh water ponds and marshes. These wetlands play a crucial role in maintaining the Buxton Woods aquifer. Rainwater running off adjacent ridges gathers in wetlands and recharge the aquifer.

    The demand for drinking water has increased with growing numbers of people on the Outer Banks. Most communities and National Park Service facilities on Hatteras Island tap into the shallow ground water reserves. Well sites and pumping rates must be monitored to avoid the danger of saltwater intrusion. In some areas, such as Ocracoke village, ground water is extremely scarce. Here, wells penetrate 600 feet into the ground before ample water reserves are found in a deeply confined aquifer. Still, an expensive process to remove salts is required before the water is drinkable.

    The Buxton Woods aquifer is the sole source of drinking water for most of Hatteras Island. No other abundant source is found on the islands between Oregon Inlet (40 miles north) and Bogue Banks (70 miles south). At the height of the summer season, Cape Hatteras National Seashore pumps up to 40,000 gallons per day from this site. The local water association, servicing adjacent villages, withdraws nearly one and a half million gallons per day during summer Cape Hatteras National Seashore is responsible for preserving and protecting the natural resources within its boundaries. Of primary concern is the protection of park wetlands found in Buxton Woods. Its diversity ranges from open marshlands, to cattail and sawgrass glades, to forested swamps. Nowhere else on the North Carolina barrier islands are such extensive wetlands found. Distinctive plant and animal communities have blossomed in such isolated settings.

    The continued survival of these wetlands balances on surface ground water levels. Plant roots must be wet for a certain length of time to survive. If ground water lowers and the soil remains dry for too long, the plants will die. Upland plants will eventually invade these sites if water levels remain low for long periods.

    Surface water levels are being lowered through withdrawal of drinking water. How much and over how wide an area it is not known. Any impacts on the aquifer and associated ecological habitats need to be identified.

    In 1991, Cape Hatteras National Seashore contracted with North Carolina State University to study the water table and surface hydrology of Buxton Woods. Investigators were to determine what impacts the pumping activities by the neighboring water plant had on the Buxton Woods aquifer. The study also intended to provide a baseline for monitoring changes caused by water withdrawal.

    Three transects containing monitoring wells have been placed in freshwater wetlands and adjacent uplands of Buxton Woods. Cape Hatteras Water Association cooperated with the Park Service in the selection of the sites. These are located in the association's present well field, the Buxton Woods Coastal Preserve which has been proposed as a future well field, and a control site approximately two miles east of the present well field. An electronic monitoring system frequently records precise water levels in each well.

    Study plots established near various well points are being examined for long term vegetation changes in response to water levels.

    More information is needed before conclusions can be reached. The Water Resource Division of the National Park Service's Washington Office allocated funds to expand the project in 1994. This new funding will allow the transects to be lengthened to encompass the entire width of the island - from ocean to sound. This expansion will allow a more complete picture of the ground water system. In order to extend the transects to the island's sound side, some private landowners will be asked to allow placement of temporary monitoring wells on their property.

    The number of vegetation study plots will be increased. Changes in plant composition will be monitored over a wider and more diverse range.

    In conjunction with this project, an NPS Water Resource Division hydrologist is studying how the aquifer stores and releases water as well as how fast water moves within the system.

    Cape Hatteras National Seashore is also cooperating with the State of North Carolina and U.S. Geological Service in a related project measuring surface water drainage from park lands. Two sites in the Buxton area are being examined; drainage culverts built in the mid 1950's along the Cape Hatteras Lighthouse entrance area; and a drainage ditch built in the 1960's and early 1970's behind Cape Point Campground.

    Islands by definition have limited resources. Though they are surrounded by vast bodies of water, the land's ability to harbor water of its own is limited. Both nature and people depend on this finite resource. Only through scientific knowledge and wise management will we maintain the delicate balance necessary to safeguard the availability of fresh water.