Quantification of Hyporheic Nitrate Removal at the Reach Scale: Exposure Times versus Residence Times

The rate of biogeochemical processing associated with natural degradation and transformation processes in the hyporheic zone (HZ) is one of the largest uncertainties in predicting nutrient fluxes. We present a lumped parameter (LPM) model that can be used to quantify the mass loss for nitrate in the HZ operating at the scale of river reaches to entire catchments. The model is based on using exposure times (ET) to account for the effective timescales of reactive transport in the HZ. Reach scale ET distributions are derived by removing the portion of hyporheic residence times (RT) associated with flow through the oxic zone. The model was used to quantify nitrate removal for two scenarios: 1) a 100 m generic river reach and 2) a small agricultural catchment in Brittany (France). For the field site hyporheic RT are derived from measured in-stream ²²²Rn activities and mass balance modelling. Simulations were carried out using different types of RT distributions (exponential, power-law and gamma-type) for which ET were derived. Mass loss of nitrate in the HZ for the field site ranged from 0-0.45 kg d^-1 depending on the RT distribution and the availability of oxygen in the streambed sediments. Simulations with power law ET distribution models only show very little removal of nitrate due to the heavy weighting towards shorter flow paths that are confined to the oxic sediments.  Based on the simulation results, we suggest that ET likely lead to more realistic estimates for nutrient removal.