THE IMPACTS OF COMMON STREAM RESTORATION TECHNIQUES ON IN-STREAM N REMOVAL: A NUMERICAL STUDY

The hyporheic exchange zone, where surface water mixes with groundwater, retains nutrients and allows for processes such as denitrification to remove excess nutrients from the stream preventing downstream export. Numerous studies have suggested a link between increased nutrient cycling and enhanced hyporheic exchange which has implications for assessing overall stream health and water quality. The objective of this study was to quantify the effects of various restoration techniques on in-stream nutrient removal. A transient storage model with Michaelis-Menten uptake kinetics was applied to simulate N removal under various stream physical and biochemical properties. The most influential stream properties on N removal were selected based on a sensitivity analysis. N removal was found to be most sensitive to velocity and in channel and storage zone uptake rates (U_c and U_s), as well as the exchange rate (α). Decreasing velocity, increasing U_c and U_s, and increasing α were found to have the greatest potential to remove N. Restoration techniques were then assessed based upon whether or not they had positive or negative impacts upon these most sensitive stream properties. Quantifying the effects of restoration techniques can better help project managers assess which techniques are most applicable for their goals and to help determine if some techniques are more harmful rather than helpful with respect to N removal. Restoration techniques such as debris dams, the addition of large woody debris, and flow path modification were recommended due to their greatest potential to enhance N removal.