Lost in Dune Translation: The Effects of Aerobic Microbial Growth Dynamics on Hyporheic Transport and Reaction in the Presence of Moving Riverbed Dunes

The hyporheic zone is the interface between surface water (SW) and groundwater (GW) in shallow aquatic sediments where reactions can attenuate contaminants. Dunes that drive hyporheic exchange in sand-bedded rivers constantly move (translate), causing “turnover exchange,” yet few numerical studies of hyporheic processes account for this motion. Furthermore, microbial communities that mediate contaminant reactions are constantly adjusting to their environments, including to effects of migrating sediment, but prior studies have not examined the combined effects of migrating dunes and microbial growth/death. We coupled SW hydrodynamics (OpenFOAM), GW hydraulics (MODFLOW), and GW reactive transport and microbial growth/death (SEAM3D) models to simulate the effects of dune translation and dynamics of aerobic microbial colonies on subsurface transport and consumption of dissolved oxygen and dissolved organic carbon (DOC). Dune translation was implemented by modifying SEAM3D to incorporate a moving frame of reference. As dune translation speed (celerity) increased with increasing SW velocity, turnover exchange, influx of DOC from SW, aerobic microbial growth, and DOC consumption all increased, given transport-limited conditions. Our no-growth models predicted only half the DOC consumption as the growth/death models despite having over six times the biomass. Explicitly simulating microbial growth/death allows simulated microbial populations to more efficiently process DOC by adjusting their spatial distribution to substrate patterns. This effect multiplies as turnover exchange increases with dune translation, highlighting the reinforcing effects of dune movement and microbial dynamics. Our results underline the importance of including both translation and growth/death dynamics when simulating hyporheic transport and reaction induced by riverbed dunes.