COASTAL RECIRCULATION OF SALTWATER DUE TO COUPLED EFFECT OF VARYING BATHYMETRY AND DENSITY

Measurements of submarine groundwater discharge (SGD) in coastal areas commonly show that the saltwater discharge is substantially greater than the freshwater discharge. Proposed saltwater exchange mechanisms include saltwater circulation driven by tidal pumping, tidal runup in intertidal areas, flow currents over bedforms, and density gradients and mixing along the freshwater-saltwater interface. We propose an additional process that may contribute to saltwater recirculation. Resistivity surveys and shallow offshore drilling and sampling show that fresh to brackish groundwater can occur beneath saline surface-water bodies in coastal environments. In the presence of local topographic relief on the seabed and ambient upward flow of fresh to brackish groundwater, the density inversion can set up a convective circulation cell with downward flow of saline porewater under the relatively lower seabed elevations and upward flow beneath the adjacent higher seabed elevations. Preliminary simulations using the SEAWAT model demonstrate the feasibility of this process. The occurrence and magnitude of the recirculation depends strongly on the values and interactions of several factors, including bedform relief, hydraulic conductivity, depth of saltwater above the seabed, and density and pressure of underlying fresher porewater. Preliminary simulations using realistic but hypothetical characteristics show that saltwater recirculation can constitute a large fraction of SGD around a bedform. This interaction between bedform topography and buoyancy forces is an additional process contributing to the observed saltwater component of SGD.