DIURNAL TEMPERATURE EFFECTS ON NITROGEN CYCLING IN HYPORHEIC ZONES

Hyporheic flow in aquatic sediment mediates the fate of nutrients and contaminants, dissolved oxygen, and temperature in the hyporheic zone (HZ). Since reactions in the HZ could produce or consume NO₃^－, the HZ could serve a nitrate source or sink role in the fluvial system. Such biogeochemical reactions in HZs are temperature dependent. However, how the dynamic temperature patterns in the HZ affect the biogeochemical function of the HZ remains unknown. To further understand this problem, we conducted a series of multiphysics numerical simulations which considered coupled turbulent open-channel fluid flow, porous media fluid flow, porous media heat transport and reactive solute transport. We assumed sinusoidally varying diurnal temperature variation for boundary conditions which represent the diurnal warming and cooling of a river. We studied the effects of different mean temperatures and different amplitudes of the diurnal temperature variations on nitrate removal or production efficiency in the HZ. The simulation results showed that the temporal area-averaged nitrification rate (NR) in the HZ follow the sinusoidal variations as the temperature pattern. Unlike NR, the temporal denitrification rate (DR) pattern depends on the mean and amplitudes of imposed temperature, and the concentrations of stream-water components. Compared to the counterparts with steady-state temperature, we found that the daily nitrate removal efficiency (DNRE) is weakly sensitive to the diurnal temperature changes, although the DNRE in the dynamic cases is proportional to the mean temperature with a small slope. The results show that while there are large variations in reactions due to dynamic temperatures over the course of a day, the time and space integrated effects may be small.