Hyporheic nitrogen removal – assessing the potential for large scale stream restoration in Sweden

Biogeochemical reactions along surface water flow paths mitigate nutrient inputs from agricultural land and can have large impacts on both the local water quality and the downstream export of nutrients from agricultural areas. Thus, stream restoration, in terms of engineered structures with the aim to increase the in-stream nutrient retention, is seen as an important strategy to restore the ecosystem functioning of degraded stream systems, mitigate excess nutrient concentrations and reduce the export to downstream recipients. Here, we propose a physically based model framework to assess the large-scale removal of Nitrogen (N) by denitrifications in the hyporheic zone along stream networks. The model framework, supported by an extensive dataset of hydromorphology and reach scale investigations, was used to estimate the current N removal in all local agricultural streams in Sweden defined as having a mean discharge < 1 m³/s and an agricultural N load > 0. Moreover, the theoretical potential to increase this removal by restoration structures that enhances the hyporheic removal efficiency and prolongs the stream residence times was assessed based on the Damköhler number, defined as the ratio between the hyporheic transport time scales and the reaction times scales.

The analyses comprised approximately 26000 stream reaches equivalent to ~75 000 km or 36% of the entire stream network in Sweden and revealed that both the N removal and the conditions limiting the hyporheic denitrification was highly dependent on the stream flow conditions. Specifically, during mean discharge conditions the aggregated results indicated that 13% of the N load to the assessed reaches was removed through hyporheic denitrification and that reaction limited conditions predominately occurred (72% of the assessed reaches). The theoretical potential of N removal, i.e. the N removal under the assumption of optimal hyporheic conditions, during mean discharge conditions was estimated to be 36% when all reaches were aggregated. Overall, the study shows that stream structures, especially if implemented over larger distances, could be a promising restoration strategy to enhance hyporheic removal and reduce terrestrial N export from agricultural areas.