Measuring and Modeling Reactive Manganese Transport in an Alpine Riverbed

Abstract:

The transport of manganese (Mn) near the river water-groundwater interface (hyporheic zone) influences river water quality in multiple ways, since the oxidation of Mn provides sorption sites for contaminant metals. To explore spatial and temporal patterns in hyporheic Mn transport using a multidisciplinary approach, we mapped relationships between hyporheic exchange, pore water chemistry, and microbial communities in the bed of East River, Colorado at high spatial resolution during summertime baseflow conditions. We found elevated dissolved Mn concentrations and more recalcitrant dissolved organic carbon (DOC) in regions of groundwater upwelling. These regions also had distinct microbial assemblages that displayed similar levels of microbial diversity to regions of river water downwelling. In seasonal observations and numerical models, oxygenated river water containing DOC mixed with groundwater rich in dissolved Mn throughout the year, but the mixing depth increased during spring snowmelt. This springtime influx of labile DOC to the hyporheic zone increased net respiration of Mn-oxides, despite an enhanced supply of dissolved oxygen. As groundwater upwelling resumed during the summertime baseflow period, the influx of Mn-rich groundwater drove net accumulation of Mn-oxides until the bed froze in winter. As climate change and water withdrawals place new pressures on groundwater discharge to alpine rivers, metal oxidation and reduction dynamics are likely to shift in riverbeds, with uncertain implications for water quality. Multidisciplinary measurements and models will be needed to unravel the trajectories of metal transport in alpine rivers.