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.

Sorption of hydrophobic organic chemicals to bacteria.

The toxicity and time-dependent sorption of three hydrophobic organic chemicals to Rhodococcus rhodochrous bacteria were investigated. In experiments, environmentally relevant concentrations of pentachlorophenol (PCP), hexachlorobenzene (HCB), and dichlorobiphenyl (DPCB) were applied to living (both growing and nongrowing) bacteria as well as to dead bacteria. For PCP (an ionizing chemical), bacterial growth decreased, and bacterial death increased, as the PCP concentration increased.