A COMPARISON OF HYDROGEOLOGIC DRIVING FORCES FOR FRESH AND SALINE GROUNDWATER DISCHARGE TO THE OCEAN

Multiple hydrogeologic and oceanographic processes can drive the exchange of fresh and saline porewaters across the seafloor (a.k.a. submarine groundwater discharge or SGD), but quantitative comparisons of the relative fluxes associated with these processes are few. This work was designed to compare fluxes associated with three hydrogeologic driving forces for SGD: fresh topography-driven flow, coastal seawater recirculation associated with the freshwater-saltwater interface and geothermal convection. Coupled fluid flow, heat transport and solute transport simulations were developed to simulate regional fluid flow in a cross-section based on a 300-km transect of North Carolina from the fall line to the base of the continental slope. Generalized rock properties were used owing to lack of data beneath the continental shelf and because this approach simplified comparison of the driving forces. Porosity and permeability decreased with depth.

In these simulations, saline SGD exceeded fresh SGD substantially. Fluxes associated with seawater recirculation were roughly equal to those of freshwater flow, and geothermal convection drove fluxes several times larger. Flow velocities in the cross-section varied over orders of magnitude even for a single rock type and played an important role in determining the geometry of the mixing zone. Saline flow systems spanned at least 20 km surrounding the coast and continental slope, suggesting that study areas for coastal flow systems should consider relatively large areas. Variations in rock properties associated with facies changes across the continental shelf and slope could alter the relative fluxes of these flow systems, but these results suggest that large-scale, long-term saline flow could contribute strongly to fluid flow in coastal settings.