Seawater circulation in sediments driven by interactions between seabed topography and fluid density
Measurements of submarine groundwater discharge (SGD) in coastal areas often show that the saltwater discharge component is substantially greater than the freshwater discharge. Several mechanisms have been proposed to explain these high saltwater discharge values, including saltwater circulation driven by wave and tidal pumping, wave and tidal setup in intertidal areas, currents over bedforms, and density gradients resulting from mixing along the freshwater-saltwater interface. In this study, a new mechanism for saltwater circulation and discharge is proposed and evaluated. The process results from interaction between bedform topography and buoyancy forces, even without flow or current over the bedform. In this mechanism, an inverted salinity (and density) profile in the presence of both a bedform on the seafloor and an upward flow of fresher groundwater from depth induces a downward flow of saline pore water under the troughs and upward flow under the adjacent crest of the bedform. The magnitude and occurrence of the mechanism were tested using numerical methods. The results indicate that this mechanism could drive seawater circulation under a limited range of conditions and contribute 20%–30% of local SGD when and where the process is operative. Bedform shape, hydraulic conductivity, hydraulic head, and salinity at depth in the porous media, aquifer thickness, effective porosity, and hydrodynamic dispersion are among the factors that control the occurrence and magnitude of the circulation of seawater by this mechanism.