BARRIER ISLAND AQUIFER HETEROGENEITIES: HATTERAS ISLAND, NORTH CAROLINA, USA

Hatteras Island is a part of the Outer Banks of North Carolina, a barrier-island chain bounded by the Atlantic Ocean and Pamlico Sound. The primary water-bearing unit on the island is the Buxton Woods Aquifer (BWA), a surficial aquifer that is approximately 24.5 meters thick. The BWA consists of three bulk units: (1) a 12-meter thick sequence of medium to coarse-grained sand with shell fragments; (2) a 3-meter thick semi-confining layer of fine-grained sand and silt; and (3) a 9.5-meter thick sequence of medium to coarse-grained sand with lenses of shell fragments. The hydraulic properties of the BWA were compiled from a 72-hour pumping test and a series of sieve analyses of core sediments obtained from six sites scattered throughout the island. The core sediments were drilled exclusively within the upper unit of the BWA. The pumping test was conducted in July, 1995, and consisted of withdrawing water from a fully-penetrating well at a rate of 400 liters per minute. Monitoring points included two fully-penetrating monitoring wells and a nest of two monitoring wells screened at the top and bottom of the aquifer. The drawdown data show a delayed yield response with the effects of stratigraphy being apparent in the late-time data. Heterogeneities within the aquifer explain the different responses. Vertical heterogeneities evident in the core data create unconfined conditions in the upper part of the aquifer and semi-confined conditions in the lower part of the aquifer. Sieve analyses of the core sediments, including sediments in the vicinity of the aquifer test site, were used to determine hydraulic properties at discrete intervals within the aquifer. The intervals were chosen based on visual lithologic changes within the core. Hazen’s approximation was used to determine hydraulic conductivities for each segment based on the grain-size distribution curves. The pumping test data indicate hydraulic conductivities of 21.2 m/d and 22.7 m/d for the bulk aquifer and upper aquifer, respectively. The hydraulic conductivities of the core samples show a lognormal distribution, with a mean and variance of 24.6 m/d and 1.56 m/d, respectively. In addition, autocorrelation functions were produced for each coring site. The correlation lengths calculated from these curves varied from 0.263 meters to 0.713 meters, with a mean correlation length of 0.484 meters.