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Effects of transient storage on solute transport and nitrogen cycling in a western U.S. river
AuthorJohnson, Zachary C.
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This dissertation describes how different types of transient storage zones affect the in-stream physical and biological transport and storage of solutes in a large stream and how this knowledge can be applied to restoration efforts. Transient storage zones provide unique habitats in streams and influence solute retention by increasing residence time and exposure to biochemically reactive surfaces. Despite the different hydraulic and biochemical characteristics of hyporheic (HTS) and surface transient storage (STS) zones, most modeling studies use a single lumped storage zone, which has likely contributed to over a decade of contradictory results regarding the relationship between transient storage and solute removal. The traditional OTIS river solute transport model was modified to include multiple storage zones (HTS and STS), calculation of uptake within each stream compartment, and Michaelis-Menten uptake kinetics. Multiple conservative and reactive tracer tests were conducted at two discharge levels in two reaches of the lower Truckee River, NV. The modified numerical model was used to fit trends in the observed data and to simulate hypothetical restoration scenarios. STS is almost 14 times more influential than HTS on the physical transport of solutes, as measured by its influence on median transport time, because of fast exchange rates between the main channel and STS zones in the lower Truckee. HTS zones were 106 times more influential in the biological retention of nitrate due to longer residence times and greater uptake rates. Results varied between sub-reaches and were dependent on a combination of geomorphology (discharge, slope, average width, average depth, and sinuosity,) and the physical and biological characteristics of the storage zones (exchange rate, size, residence time, and uptake rate). This work is unique in describing the physical and biological characteristics of both the hyporheic and surface transient storage zones in a large stream (> 0.5 m<super>3</super> s<super>-1</super>). With only physical retention information, STS appears to dominate in-stream transient storage processes. However, when both physical and biological processes are taken into account it becomes apparent that HTS dominates in-stream nitrogen removal. Results suggest that separation of the two transient storage zones is needed for a complete understanding of solute transport, storage, and removal. This information can help guide stream restoration activities. By simulating hypothetical restoration scenarios, this research shows that the size of the hyporheic transient storage zone is the most important factor for removing N via denitrification and that a combination of restoration targets--including increased width-to-depth ratio, sinuosity, and hyporheic size--produces more efficient N removal than the sum of the removal from individual restoration targets. In-stream N removal can also be increased by adjusting the fraction of organic versus inorganic N from a point source. A visual tool depicts the potential for stream restoration activities to significantly increase N removal via denitrification using the combination of hyporheic size, maximum denitrification rate, and half saturation concentration for denitrification.