Impact of Intra-meander Hyporheic Flow on Nitrogen Cycling

Abstract Redox gradients within hyporheic zones significantly impact the biogeochemical cycling of carbon and nitrogen. To investigate the effect of these redox gradients on nitrogen transformation in the subsurface, we integrated a genome-informed complex reaction network into PFLOTRAN, which is an open source, massively parallel, three-dimensional, reactive flow and transport code. This study was conducted in the lower East River catchment in southwestern Colorado. The lower East River has multiple river meanders extending over a distance of 11 km in rolling to mountainous region of the East Taylor watershed. We carried out reactive flow and transport simulations within two stream meanders to describe the biogeochemical zonation that evolves due to upwelling of nutrient rich groundwater and downwelling of oxygen rich stream. The specific objectives were to examine (1) the effect of hyporheic flow on biogeochemical zonation and (2) how meanders affect local nitrogen fluxes and transformation. Simulation results demonstrate that hyporheic flow paths within intra-meander regions lead to lateral redox zonation, which significantly impact nitrogen export into the stream system. Also, meander-driven hyporheic flow paths enhance denitrification rates because of the extended hyporheic region.