Impacts of Municipal and Commercial Usage of a Shallow Unconfined Aquifer, Nags Head Area Maritime Forest and Wetlands, Atlantic Coast Barrier Islands, North Carolina

Abstract

A three-dimensional finite-difference digital model was used to simulate groundwater flow within the Nags Head area maritime forest and wetlands. The study area is 16 km2 (4000 acres) on the North Carolina Barrier Island. The unconfined aquifer is part of the North Carolina Coastal Plain Aquifer System. The model was developed from a hydrogeologic framework that is based on geology, sediment analysis, aquifer tests, high resolution groundwater analysis, input fluxes, and appropriate boundary conditions. The model was calibrated by comparing observed and simulated water levels. The model calibration was achieved by adjusting model parameters such as hydraulic conductivity, recharge, and storativity until the simulated water levels were within 0.5 feet (15 cm) of observed water levels. Analysis indicates that the model is sensitive to decreases in hydraulic conductivity. The model is only slightly sensitive to changes in storativity, recharge, increased hydraulic conductivity, and vertical hydraulic conductivity. Precipitation is the only source of recharge to the unconfined aquifer, and it averages approximately 12 inches/year (30.5 cm/yr.). Flow is primarily horizontal with the water table exposed as shallow ponds and puddles within inter-dunal lows. Hydrologic analyses of the flow system, using the calibrated model, indicate that pumping conditions from Fresh Pond create a cone of depression around the pond and alter the groundwater flow. The cone of depression impacts the surrounding areas, especially west of Fresh Pond, and is magnified during times of reduced recharge to the aquifer. Declines of groundwater levels, which are a result of water taken from Fresh Pond, are extensive in some areas and minimal in other areas. Decline of hydraulic heads of more than 2 to 4.5 feet (0.6 to 1.4 meters), depending on temporal variability in recharge and pumping rates, have been predicted from the calibrated model. The area with the most significant impacts is west of Fresh Pond at the groundwater divide within Nags Head Woods Ecological Preserve. Other areas impacted are to the east and south of Fresh pond. Artificial recharge from a small sewage treatment plant near Nags Head could be pumped to an appropriate engineered wetland and provide some limited benefits to the aquifer. Additional commercial wells pumping groundwater east of Fresh Pond provide additional drawdown to the pond area water table.