Nitrogen cycle in gravel bed rivers: The effect of the hyporheic zone

Several studies have showed the importance of microbial processes in removing nitrogen from rivers with first-order gravel bed streams playing a major role in reducing the total load. Consequently, modeling transport of nutrients and contaminant in rivers calls for including temporary storage and nitrificationdenitrification processes within hyporheic and riparian zones. In gravel bed rivers alternate zones of high and low pressure generated by bedforms induce a complex flow pattern within the hyporheic zone that interacts with the stream through downwelling and upwelling zones. We solved the two transport equations for NH4 and NO3, coupled with the chained first-order kinetics modeling nitrification and denitrification processes within the hyporheic zone, by using a Lagrangian approach and assuming that dispersion is negligible. With this simple, yet powerful, model we studied the interplay between streambed morphology and nitrogen export from hyporheic zone of a gravel bed river including the interplay between ammonia and nitrate.