Source of saltwater intrusion in the Castle Hayne aquifer on the western part of Bogue Banks barrier island, North Carolina

Abstract

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 eleven municipal wells that are solely screened in the Castle Hayne aquifer on the central and western parts of the island, three wells withdraw water with elevated chloride levels that exceed the drinking water standards of the United States Environmental Protection Agency. The wells on the western side of the island have the highest chloride levels (>1,700 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 elevated chloride concentrations in the potable water source. Despite this development, the underlying source of saltwater intrusion into the Castle Hayne aquifer is still not well known. Therefore, the goal of this study was to determine the source of 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 were used to design a three dimensional multi-layered, multi-density groundwater flow model using visual MODFLOW. The model was used in conjunction with chloride and long-term water level records to test the hypotheses that the source of saltwater intrusion into the Castle Hayne aquifer is from surface waters surrounding the island and/or a paleochannel underlying the island. Results from modeling suggest that saltwater emanating from surface waters surrounding the island is being drawn into municipal wells and not from the paleochannel underlying the island. The model suggests that brackish surface waters from Bogue Sound migrate downwards through overlying aquifers into the Castle Hayne aquifer. The major driver of the migration is groundwater withdrawal from the municipal wells on the western part of the island. Understanding how groundwater withdrawals impact the migration of saltwater in the Castle Hayne aquifer arms water managers with the knowledge to better safeguard the major source of potable water on the island for future generations.