MECHANISM FOR SALTWATER INTRUSION IN A CONFINED KARST AQUIFER ON THE WESTERN PART OF BOGUE BANKS BARRIER ISLAND, NORTH CAROLINA

Saltwater intrusion is occurring in the confined Castle Hayne aquifer on the western part of the east-west trending barrier island of Bogue Banks, North Carolina. Of 12 production wells that are screened in the Castle Hayne aquifer on the central and western parts of the island, three wells withdraw water with high chloride levels. The western most wells have the highest chloride levels (>1700 mg/L), whereas the levels progressively decrease in the wells to the east (<250 mg/L). Because the Castle Hayne aquifer is the sole source of potable water on the island, a reverse osmosis plant was built on the western part of the island to address the saltwater intrusion problem. Despite this development, the underlying mechanism that is driving saltwater intrusion into the Castle Hayne aquifer is still not well understood. Therefore, the goal of this study is to determine the mechanism causing saltwater intrusion in the Castle Hayne aquifer on the western part of Bogue Banks. Borehole logs, seismic profiles, vibracore, and ground penetrating radar data collected by other researchers are used to build a 3D multi-layered, stratigraphic model. The 3D model is then used in conjunction with long-term chloride and water level records to test the hypotheses that salt levels are high in groundwater wells because (a) saltwater is being withdrawn from below the saltwater-freshwater interface in the Castle Hayne aquifer, (b) saltwater emanating from Bogue Inlet is being drawn into production wells, and (c) saltwater originating from a paleochannel is being drawn into production wells. The results suggest that (a) production wells in the Castle Hayne aquifer are well above the saltwater-freshwater interface, (b) Bogue Inlet does not incise the upper Castle Hayne confining unit, and (c) a paleochannel breaches the upper confining unit of the Castle Hayne aquifer up-gradient of the island. Owing to the location and hydraulic conductivity of the paleochannel on the western end of the island, the paleochannel is possibly the conduit by which saltwater intrusion is occurring in the Castle Hayne aquifer. Identifying the mechanism causing saltwater intrusion on the western part of the island would arm water managers with the knowledge to better safeguard the major source of potable water on the island for future generations.