FLUVIAL CHANNELS OFFSHORE NORTH CAROLINA: A ROUTE FOR GROUNDWATER DISCHARGE ?

The hydrology around Wrightsville Beach, North Carolina is dominated by the Castle Hayne aquifer, a heterogeneous limestone that is buried approximately 20m deep at the shoreline beneath the surface beach sands and an Oligocene sequence of fine grained sands and silts.

We have collected a series of offshore chirp seismic and electromagnetic data that constrain the structure of the seafloor into the Castle Hayne. Our data show a series of fluvial channels that incise the Oligocene sequence and that in places appear to intersect the top surface of the limestone.

Based on the complex geophysical signature of these channels, we hypothesize that they act as high permeability conduits, allowing groundwater to flow from the limestone up to the seafloor. If this is true, then it has important implications for groundwater discharge and nutrient supply to the seafloor. To test our hypothesis, we have constructed a hydrologic model using the physical properties measurements made during the geophysical survey as constraints. We first of all constructed a regional model to constrain head boundary conditions and then have used these in a more detailed local 2D model including a fluvial channel. We will present the results of our modeling and discuss implications for groundwater discharge to the continental shelf.