A numerical investigation of the potential impact of stream restoration on in-stream N removal

• Two-zone storage model of stream restoration using Michaelis–Menten kinetics.
• Hyporheic size and maximum denitrification rate were most important parameters.
• More efficient N removal is achieved when combining restoration targets.
• The relative inorganic–organic N fraction of input affects potential removal.
• Graphs indicate potential effectiveness of restoration activities to remove N.

Wastewater treatment plants are common point sources of nutrients to streams. Excess loading of nutrients, particularly nitrogen (N), can result in significant water quality degradation. Where stream loading cannot be increased by water quality trading with other point or nonpoint sources, stream restoration may be an alternative means for point sources to increase loading while maintaining or improving stream health by increasing in-stream N removal via denitrification. However, the primary drivers of nitrogen removal are currently not well understood and thus optimizing restoration efforts is difficult. A two-storage zone transport model with Michaelis–Menten uptake kinetics was applied to a river system based on the Truckee River of Nevada to simulate N removal for multiple restoration scenarios and different types of nitrogen loading. Rates of N removal were found to be most sensitive to the size of the hyporheic zone (AHTS) and maximum denitrification rate in the hyporheic zone (U′max,HTS), followed by the half-saturation concentration for denitrification (Km,HTS). Graphs that incorporate the ranges of these three parameters indicate the potential effectiveness of restoration activities for increasing N removal. Combining restoration targets (e.g., increasing AHTS, sinuosity, width-to-depth ratio, etc.) provided more N removal than the sum of N removal from the individual targets. The relative fractions of the three nitrogen species (dissolved organic-N, total ammonia-N, and nitrate-N) was found to significantly affect a stream's potential to remove N. Together, these results can be used to help guide stream restoration activities to increase a stream’s N removal capacity.