Nitrate removal efficiency in hyporheic zones : the effect of temperature and bedform dynamics
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Fundamental understanding of bedform-induced hyporheic flow and how it controls the nitrate dynamics in the hyporheic zone (HZ) is critical for environmental and ecological problems, such as eutrophication, deterioration of water quality, and loss of biodiversity. The HZ is regarded as a biogeochemical hotspot for removing nitrate from the river system. Although hyporheic flux has been extensively studied in the HZ in the last decade, the cycle of nitrate dynamics in the HZ is still poorly understood. To better characterize and predict nitrate cycling in the HZ, I have investigated the role of HZ in removing nitrate in response to temperature variations and bedform migration through numerical experiments. I have designed and generated all bedform shapes for this dissertation. All numerical experiments followed the same sequential procedures: (1) Solving Reynolds-averaged Navier-Stokes equations to obtain pressure distribution along the sediment-water interface; (2) The bedform-induced pressure along sediment-water interface drives water entering into and out of HZ by solving the Darcy’s equation; (3) Meanwhile, the transport equations were implemented for solving reactive species and/or temperature distributions, depending on whether temperature was assumed to be spatially heterogeneous or homogeneous. The reactive species were either consumed or produced that was closely related to the reaction chains for the nitrate transformations in the HZ. Here, we only considered aerobic respiration, denitrification, and nitrification for nitrate cycling in the HZ. My dissertation started with the simplest cases assuming uniform temperature in the HZ. I found that nitrate transformations in the HZ are temperature-dependent since the chemical reaction rates increase with enlarging temperature. The functionality of HZ acting as nitrate source or sink depends strongly on the stream water quality. When the HZ serves as nitrate sink, the nitrate removal efficiency increases with temperature. Moreover, since temperature changes diurnally following a sinusoidal function, a persistent biogeochemical hotspot for removing nitrate is present regardless of the occurrence of dynamic and complex hyporheic temperature patterns. The daily-averaged nitrate removal efficiency with instantaneously changing temperature is fairly identical to the counterpart by using uniform temperature in the HZ. Last but not least, I generated more realistic moving ripple bedforms. The migration rate of ripples causes different hyporheic flux and thus reactive transport processes in the HZ. I found that the nitrate removal efficiency increases asymptotically with Damkohler number, and the immobile ripples overestimate the nitrate removal efficiency compared to that for mobile ripples. All above-mentioned research results can be readily extended for large scales.