Nutrient cycling in dune-induced hyporheic exchange of reactive solutes

AGU 2011 Fall Meeting: abstract & oral presentation. Abs: The hyporheic zone is an ecotone connecting the stream and groundwater ecosystems that plays a significant role for stream biogeochemistry. Hyporheic water exchanges across the stream-sediment interface occur generally in response to variations in bed topography, with a very wide range of spatial and temporal scales. For instance, small-scale exchanges are mainly induced by river bed forms, like ripples and dunes, while large-scale exchanges depend on larger geomorphological features. The chemicals enter the sediments with the water and they are transformed into oxidized or reduced substances by biogeochemical reactions, mediated by hyporheic microfauna. These sediment-scale transformations have an influence on the quality of the upwelling water and potentially also on the quality of the stream water. Thus, the interaction of hyporheic fluxes and biochemical reactions in the streambed concur to affect subsurface solute concentrations and eventually nutrient cycling in the fluvial corridor. In this work we investigate the interplay of hydrological and biochemical processes in a duned streambed and their effect on spatial distribution of solutes. We employ a numerical model to simulate the turbulent water flow and the pressure distribution over the dunes, and then to evaluate the flow field and the biochemical reactions in the hyporheic sediments. We take into account for representative reactive compounds: dissolved organic carbon (DOC), oxygen (O2), nitrate (NO3-) and ammonium (NH4+). Sensitivity analyses are performed to analyze the influence of hydrological and chemical properties of the system on solute reaction rates. The results demonstrate the effect of stream velocity and sediment permeability on the chemical zonation. Increasing sediment permeability as well as stream velocity enhances the reaction rates. Stream water quality is also shown to influence the reactive behavior of the sediments. In particular, the availability of dissolved organic carbon determines whether the streambed acts as a net sink or source of nitrate. This study represents an initial step for a better understanding of the complex interactions between hydrodynamical and biochemical processes in the hyporheic zone