PNNL

Abstract

Exchange processes along river corridors play an essential role in ecosystem services, providing thermal, nutrient, and water quality regulation in streams. In meandering rivers, the exchange is driven by geomorphic features of the sinuous rivers and modulated by regional groundwater fluxes. This exchange process operates as a biochemical reactor at multiple scales, directing the fate and transport of solutes, energy, and matter, thus regulating the water quality of the stream. One critical biogeochemical process in water quality regulation is nitrogen cycling. Nitrogen transformation is controlled by the availability of other chemical constituents, like oxygen and dissolved organic matter and the rate at which the biochemical process occurs. Therefore, gaining a mechanistic understanding of the role of hydrogeomorphic drivers and modulators in this hyporheic exchange is essential for managing and assessing restoration strategies. With the latter in mind, we performed a systematic analysis of sinuosity-driven hyporheic exchange. We use a simplified groundwater flow and transport model to explore the role of meander geometry and regional groundwater fuxes on the hydrodynamics and biogeochemical potential of meandering streams. We explore the flow field dynamics, residence time distribution, and nitrogen cycling in four distinct meander geometries and various biogeochemical settings for polluted and pristine streams. Our results show that the meander shape is an essential hydrodynamic control, where short meander necks limit the compression effect of regional groundwater flow, creating larger hyporheic fluxes. However, the narrowing of the meander decreases the residence times of the hyporheic exchange, reducing its effectiveness in denitrifying the river. Furthermore, the denitrication process is also limited by the amount of dissolved and particulate organic carbon in streams and the hyporheic zone, transforming meanders into potential sources of nitrates. This work provides important insight into the biogeochemical potential of meanders in nitrogen cycling. Also, this modeling and prediction tool can be used for water quality management and restoration strategies.