LARGE-SCALE HYDROGEOLOGIC FLOW SYSTEMS IN COASTAL SETTINGS

Studies of groundwater flow in coastal aquifers often focus on freshwater resources and investigate flow in a narrow (<5 km) zone surrounding the coastline. In this work numerical flow and transport models were used to explore coastal flow in a regional context and to compare fresh and saline submarine groundwater discharge (SGD) for regional flow systems. The models were used to estimate SGD associated with topography-driven flow, seawater recirculation, and geothermal convection in a passive margin setting. Land surface and seafloor topography of the simulation domain were based on two cross-sections of North Carolina. Data for the hydraulic properties of the continental shelf are very sparse, so sensitivity studies were used to explore the effect of varying hydraulic and transport parameters. In the simulations, the flow systems contributing to SGD spanned at least 20 km surrounding the coast and continental slope. Studies of fresh groundwater resources may be justified in using smaller study areas, but studies of SGD may need to consider large areas. In a simulation using hydraulic properties representative of a silty sand and an anisotropy ratio of 100, geothermal convection generated SGD of 22,000 kg/yr, which exceeded topography-driven flow by a factor of 1.5, and the flux associated with seawater recirculation was a factor of 10 smaller than the geothermal flux. Over the 320 km stretch of coastline between Cape Fear NC and Savannah GA these simulations suggest SGD of the order of 10¹⁰ kg/yr. River discharge for that area is of the order of 10¹³ kg/yr, but the flux of solutes is much higher in saline fluids than in fresh river water. Results suggest that saline flow associated with seawater recirculation and geothermal convection should be considered in studies of SGD.