Exchange across a sediment–water interface with ambient groundwater discharge

SummaryWe investigate the interaction between turbulent water-column flow, current-topography-driven flow in underlying permeable sediments, and ambient groundwater discharge (AGD) from/to deep groundwater. Turbulent flow in the water column is simulated using a k–ω model and linked to a Darcian model of pore-water flow. AGD reduces the spatial extent of the current-topography driven interfacial exchange zone (IEZ) within the sediments and prevents its development when turbulent flow cannot induce sufficient pressure gradients along the sediment–water interface (SWI) to overcome AGD. A Morgan–Mercer–Flodin-type model describes how the presence and size of the IEZ depends on water-column Reynolds number (Re) and AGD, while the IEZ flux dependence is described by a power model. The mean residence time of water flowing through the IEZ is large at low Re, when the IEZ is of limited spatial extent, decreases at higher Re when the IEZ is more extensive, and decreases with the presence of AGD. Although metrics for interfacial exchange (size, flux and mean residence time) look similar for both upward and downward AGD, the geometry of the IEZ is different. For upward AGD, the IEZ is centered on the bottom-pressure maximum along the stoss face of dunes, near where the water-column eddy reattaches. Deep groundwater upwells near the bottom-pressure minimum which is located at the crest. Under downward AGD, the IEZ forms around the pressure minimum at the crest. The water column downwells near the bottom-pressure maximum along the stoss face, some water downwelling deep into the sediments and not returning to the SWI.