Effects of high-resolution topography and hydraulics on reach-scale estimates of nitrate transport and retention in the Suncook River, Epsom, NH

Meghan Arpino, University of New Hampshire, Durham

Abstract

River networks provide the valuable ecosystem service of mitigating the transport of excess nitrate downstream. Increased nitrate uptake may occur in hot spots, such as side pools, or during hot moments, including periods of low flow. In this study, a 2D hydraulic model was developed using high-resolution green light detection and ranging (LiDAR) bathymetry and the Delft3D-Flow hydrodynamic and water quality modules. The model was used to determine nitrate transport characteristics for a 4.5-km-long reach of the Suncook River in Epsom, New Hampshire, for a range of low to moderate discharges. Model output was validated using non-toxic dye releases as well as water surface elevation and velocity point measurements. Nitrate uptake potential was simulated using three scenarios which assumed 1) a spatially uniform uptake rate constant, 2) a spatially varied uptake rate constant and uniform uptake velocity, and 3) both spatially varied uptake rate constant and uptake velocity. Longitudinal dispersion generally increased with discharge, due in part to the increased inundation of channel margins. Reach-scale nitrate retention was lower at higher discharges, due in part to reduced detention time in the reach, and was relatively similar across uptake scenarios. This study provided estimates of effective reach-scale nitrate retention accounting for both patch-scale uptake rates and hydraulic connectivity, which will be suitable for incorporation into network-scale models used to manage the disturbance caused by excess nitrate in riverine ecosystems.