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the top of the food chain, the secondary carnivores or predators. The bioaccumulation factor (and therefore, biomagnification) can be related to the bioconcentration factor by a food chain multiplier (FM):

The U.S. EPA recommends using food chain multipliers based on KOW and trophic level as shown in Table 18.6. At log KOW values higher than those in the table, the FM becomes uncertain and may even decrease due to slow transport kinetics and bioavail-ability. Thus, if log KOW is 5.0, the bioaccumulation factor for carnivores (trophic level 3) will be about twice the bioconcentration factor.

Because organochlorine pesticides are not only lipophilic and persistent but also toxic by their nature, they are the substances most capable of causing harm by biomagnification. Furthermore, biomagnification limits the effectiveness of pesticides. Since in agriculture pesticides are often used to control plant pests, they are applied at levels toxic to the base of the food pyramid. If the pesticide biomagnifies, this will necessarily result in more harm to predatory insects, which are the ones needed to control the pests. Thus, ironically, it was sometimes found that applications of these substances resulted in increased pest problems. For these reasons, organochlorine pesticides have been replaced in many applications by other pesticides that do not biomagnify.

18.7.5 Multicompartment Models

If sufficient information is available, or if contaminant behaviors that are more complex are to be studied, multiple-compartment models can be used. For example, Figure 18.10 shows a model that could be used to predict narcosis by inhalation of a hydrocarbon. A separate mass-balance equation must be written for each compartment. Each arrow represents a flux, and a term must be added to the liver compartment for elimination by biotransformation. Although it may not be possible to define a half-life or a bioaccumulation factor for some of the more complex multicomponent systems, their behavior often may be approximated by one of those parameters.

Figure 18.10 Multicompartment model of an animal.

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