In shallow, fetch-limited estuaries, variations in current and wave energy promote heterogeneous surface water-groundwater mixing (benthic exchange), which influences biogeochemical activity. Here, we characterize heterogeneity in benthic exchange within the subtidal zone of the Delaware Inland Bays, a satellite site of the CRB CZO, by linking hydrodynamic circulation models with mathematical solutions for benthic exchange forced by current-bedform interactions, tides, and waves.

Benthic fluxes oscillate over tidal cycles as fluctuating water depths alter fluid interactions with the bed. Current-bedform interactions drive rapid exchange in well-circulated channels. Wave pumping drives rapid exchange in downwind shoals. During high-energy storms, simulated wave pumping rates increase by orders of magnitude, demonstrating the importance of storms in solute transfer through the benthic layer. In moderate wind conditions, integrated benthic exchange rates due to wave, current, and tidal pumping are each one-third of runoff rates to the estuary and one-quarter of fresh groundwater discharge rates. Benthic exchange is thus a significant and dynamic component of an estuary’s fluid budget that may influence estuarine carbon cycles.