The sediment-water flux of HOCs due to "diffusion" or is there a well-mixed layer? If there is, does it matter?

In most present modeling of the sediment-water flux of hydrophobic organic chemicals (HOCs), the flux due to sediment erosion/deposition is described explicitly by means of a sediment transport model, while the remaining components of the flux (molecular diffusion, bioturbation, and groundwater flow) are lumped together and modeled as a "diffusive" flux. This diffusive flux is usually described by means of a mass transfer approximation with the implicit assumption of a well-mixed contaminant layer of thickness h in the sediments. On the basis of recent experiments and theoretical modeling, the justification forthis assumption and the quantification of this diffusive flux are discussed here. In particular, for HOCs with large partition coefficients, it is argued that a well-mixed layer often may not exist, and, when it does, it is slow to form; that h is difficult to define and even harder to quantify; and that, as far as long-term predictions (up to 100 years) of this diffusive flux are concerned, the exact value for h probably does not matter. What does matter are the magnitudes and time dependencies of each of the components of the flux and the interactions between the diffusive flux and the flux due to erosion/deposition.