Modeling the Bioconcentration of Organic Compounds by Fish:  A Novel Approach

Bioconcentration is generally assumed to be similar to the partitioning of compounds between an aqueous and lipoid phase. In the case of bioconcentration lipoid phase is thought to consist of the lipid and lipoid tissues of an organism, e.g. a fish. However, it is frequently observed that for highly lipophilic compounds, defined as compounds with a high octanol-water partition coefficient, or log K-ow, that simple view seems to break down. At low and intermediate log K-ow there seems to he a linear relationship between bioconcentration, expressed as log BCF, and log K-ow. At high log K-ow deviations are seen, generally indicating that for these compounds BCF increases less than proportionally with increasing log K-ow. Many different explanations and accompanying correlation models have been proposed to date. We present here a model that is capable of explaining the different shapes of the BCF/K-ow relationships empirically observed, based on the effect of the presence of tiny amounts of different types of organic material in the aqueous phase. The actual shape of the nonlinear curve simply depends on the amount of organic material in the water, the difference in "affinity" for hydrophobic compounds of the organic material in water and lipid in fish, and to a lesser extent the amount of lipid in fish. The model presented also fits a large dataset of experimental BCF values with fitted parameter values that are consistent with the biology and physical chemistry of the described system.