Vertical Hydrological Exchange Flows Control Methane Emissions From Riverbed Sediments

CH4 emissions from inland waters are highly uncertain in the current global CH4 budget, especially for streams, rivers and other lotic systems. Previous studies have attributed the strong spatiotemporal heterogeneity of riverine CH4 to environmental factors such as sediment type, water level, temperature, or particulate organic carbon abundance through correlation analysis. However, a mechanistic understanding of the basis for such heterogeneity is lacking. Here we combine sediment CH4 data from the Hanford reach of the Columbia River with a biogeochemical transport-reaction model to show that vertical hydrologic exchange flows (VHEFs), driven by the difference between river stage and groundwater level, determine CH4 flux at the sediment-water interface (SWI). CH4 fluxes show a nonlinear relationship with the magnitude of VHEFs, where high VHEFs introduce O2 into riverbed sediments, which inhibits CH4 production, and low VHEFs causes transient reduction in CH4 flux (relative to production) due to reduced advective CH4 transport. In addition, VHEF leads to the hysteresis of temperature rise and CH4 emissions because high river discharge caused by snowmelt in spring leads to strong downwelling flow that offsets increasing CH4 production with temperature rise. Our findings reveal how the interplay between hydrologic flux and microbial metabolic pathways that compete with methanogenic pathways can produce complex patterns in CH4 production and emission in riverbed alluvial sediments.