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Molecular diffusion, dispersion, and turbulent diffusion can all contribute to the transport of mass across the sediment‐water interface (SWI). Flow observations across the SWI are critical in the description of these transport processes but are currently very limited as the sediment bed is inaccessible in most experimental approaches. We overcome this constraint by combining refractive‐index matching (RIM) with particle‐tracking velocimetry (PTV). Using measurements of the flow dynamics across the SWI, we propose a mechanistic model that describes the interfacial mass transport over a wide and relevant parameter space. We show there are three transport regimes where molecular, dispersive, or turbulent transport dominates the mass flux of a passive tracer across the SWI. Transitions between these regimes are defined by the permeability Reynolds number , where K is the sediment permeability, is the shear …