A shift to upwelling conditions over the subsequent baseflow period allows for groundwater rich in dissolved Mn to mix with oxygenated river water in the shallow subsurface, resulting in net accumulation of Mn-oxides until the bed freezes in winter. In field observations and models, dissolved Mn is flushed from the streambed during spring snowmelt. Reactive transport models were developed to understand seasonal changes in Mn cycling in the streambed because Mn oxides influence the mobility of other metals in streams. We also observed an increase in Mn concentrations within deeper streambed sediments during the baseflow season. Conversely, we demonstrated that depth-resolved microbial communities became more distinct and stratified across a depth gradient during periods of low- and base-flow when upwelling groundwater exerted more influence in the riverbed. We were able to measure a dominant period of downwelling river water associated with high river discharge in Spring 2017 that led to the mixing of microbial communities across a 60-cm depth profile through the riverbed at three locations around the meander, and higher rates of aerobic respiration via delivery of dissolved oxygen (O2) and dissolved organic carbon (DOC). Further, we performed high more » spatial resolution sampling to capture heterogeneous mixing patterns around a characteristic meander on the river and developed reactive transport models to explain seasonal patterns of manganese (Mn) cycling in riverbed sediments. Through this work, we aimed to quantify upwelling and downwelling fluxes across seasons and assess the impact of these dynamics on microbial community assembly and geochemical gradients at East River, Colorado. Seasonal hydrology controls the expansion and contraction of hyporheic zones, with greater downwelling oxic river water influence during periods of peak river discharge (linked to snowmelt), and greater influence of upwelling anoxic groundwater during periods of low- and base-flow. Hyporheic zones (where surface water and groundwater mix in streambeds) play a critical role in the movement of chemicals within watersheds, particularly in upland headwater streams. The simulation results further indicated that the meander acted as a sink for organic and inorganic carbon as well as iron during the extended baseflow and high-water conditions however, these geochemical species were released into the river during the falling limb of the hydrograph. The sensitivity analysis results showed that permeability had a more significant impact on biogeochemical zonation more » compared to the reaction pathways under transient hydrologic conditions. The simulation results further demonstrated that the reductive potential of the lateral redox zonation was controlled by groundwater velocities resulting from river stage fluctuations, with low-water conditions promoting reducing conditions. Consistent with field observations, simulated dissolved oxygen and nitrate decreased along the intrameander flow paths while iron (Fe2+) concentration increased. The model was able to capture the field-observed trends of dissolved oxygen, nitrate, iron, pH, and total inorganic carbon along a 2-D transect. The meander's overall contribution to the river was quantified by integrating geochemical outfluxes along the outside of the meander bend. Two-dimensional reactive flow and transport simulations were performed (1) to evaluate how transient hydrological conditions control the lateral redox zonation within an intrameander region of the East River in Colorado and (2) to quantify the impact of a single meander on subsurface exports of carbon and other geochemical species to the river. To understand how redox processes influence carbon, nitrogen, and iron cycling within the intrameander hyporheic zone, we developed a biotic and abiotic reaction network and incorporated it into the reactive transport simulator PFLOTRAN.
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