GROUNDWATER RESOURCE MANAGEMENT AND SALTWATER INTRUSION MITIGATION, ROANOKE ISLAND, DARE COUNTY, NORTH CAROLINA

Coastal Plain aquifers are prone to saltwater contamination due to various natural and anthropogenic factors. We present a case study of saltwater contamination of semi-confined aquifers beneath Roanoke Island, Dare County, North Carolina. Roanoke Island is underlain by more than 1900 meters of marine and estuarine sediments. The upper 50 meters include Pleistocene and Holocene sediments that generally contain fresh pore water. Prior studies delineated three fresh-water aquifers, including: the Surficial, Upper Principal, and Lower Principal Aquifers.

Water demands from increased resident population and tourism prompted Dare County to construct the Skyco wellfield and water treatment plant (WTP) in 1979. The wellfield includes 9 water-supply wells open to the Upper and Lower Principal Aquifers that produce 16.28 megaliters/day of fresh water to the WTP. The wellfield has caused a local cone of depression that has reversed the natural vertical hydraulic gradient between the semi-confined Principal Aquifers and the overlying unconfined Surficial Aquifer. Depressurization of the Principal Aquifers has increased the potential for saltwater to migrate downward into these aquifers from local saltwater sources. Southern portions of Roanoke Island near Wanchese have experienced chronic saltwater contamination of poorly-constructed private wells. Dare County has replaced numerous contaminated private wells, but this response has been expensive, and it has not provided a permanent solution to the problem.

We analyzed the hydrostratigraphic framework beneath Roanoke Island by evaluating stratigraphic and geophysical well logs, water levels, and water-quality data. Maps and cross sections were developed to depict aquifer elevations, transmissivity, head, and chloride concentrations. Several mechanisms for saltwater intrusion were identified. New management strategies were developed, including: increased wellhead protection measures, extension of public water service and abandonment of private wells in Wanchese, and exploration of deeper brackish aquifers as potential new sources of water. These management strategies will help Dare County to mitigate saltwater intrusion and to provide more sustainable use of the limited fresh groundwater resources beneath Roanoke Island.