UC Irvine

Bioavailable forms of nitrogen, such as nitrate, are necessary for aquatic ecosystem productivity. Excess nitrate in aquatic systems, however, can adversely affect ecosystems and degrade both surface water and groundwater. Some of this excess nitrate can be removed in the sediments that line the bottom of rivers and coastal waters, through the exchange of water between surface water and groundwater (known as hyporheic exchange).

Several process-based models have been proposed for estimating nitrate removal in aquatic systems but these (1) do not consider the multiscale nature of hyporheic exchange flows; (2) rely on simplified conceptualizations of mixing within streambed sediments (e.g., a well-mixed box); (3) neglect important steps in the N-cycle (e.g., nitrification and ammonification); and/or (4) adopt pseudo-first-order kinetic descriptions of denitrification. On the other hand, a number of empirical correlations have been published based on in-stream measurements of nitrate uptake using reach-scale stable isotope tracer experiments. While these correlations are noteworthy in many respects, they do not account for physical processes known to play an important role in nutrient processing, such as the exchange of water between a stream, sediments, and groundwater.

In this thesis, I develop and test a simple and scalable process-based model for estimating the nitrate uptake velocity that addresses the limitations identified above. In particular, my model accounts for: (1) hyporheic exchange at multiple scales together with ambient groundwater flow; (2) the broad residence time distributions characteristic of hyporheic exchange; (3) key biogeochemical reactions associated with N-cycling, including respiration, ammonification, nitrification, and denitrification; and (4) the nonlinear nature of the pertinent biogeochemical reaction rates, including Monod kinetics for aerobic respiration and denitrification, and second-order kinetics for nitrification. Using this modeling framework I systematically evaluate primary controls on stream N-cycling and evaluate how multi-scale and regional factors are likely to affect this process. I also demonstrate how my model predictions compare with previously published reach-scale measurements of nitrate removal, develop scaling relationships by which my process-based model can be applicable to larger scales (i.e., watershed and regional scales), and provide some mechanistic explanations for previous observations.