Modeling the Sediment-Water Flux of Hydrophobic Organic Chemicals Due to Bioturbation

Of the three processes (sediment erosion/deposition, molecular diffusion, and bioturbation) that primarily govern the sediment-water flux of hydrophobic organic chemicals (HOCs), bioturbation is probably the least well understood and quantified. The reason is the large variety of benthic organisms which may be present in surficial sediments and the different ways in which they affect the sediments. This is further complicated by the effects of non-equilibrium sorption which is significant for HOCs with high partition coefficients. In order to understand and quantify the flux due to bioturbation, laboratory experiments have been done with three benthic organisms (Lumbriculus variegatus, a vertical burrower and an oligochaete; Chironomus tentans, a horizontal burrower; and Hyalella azteca, a mechanical mixer of surficial sediments) and an HOC (hexachlorobenzene) in Lake Michigan sediments. Each of these organisms acts on the sediments in a different way; as a result, they are representative of a wide range of benthic organisms found in the sediments of surface waters. A convection-diffusion model including non-equilibrium sorption has been developed in order to analyze these experiments. Good agreement between the experimental and calculated results is obtained. The model is quite general and should be applicable to other organisms, HOCs, and sediments with the appropriate change of parameters.