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. 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.