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(321) Limitations of equilibrium partitioning for estimating bioavailability to earthworms: lack of equilibrium and implications of feeding. Jager, Tjalling1, Peijnenburg, Willie1, Hermens, Joop2, Van Leeuwen, Cees1, 1 2 ABSTRACT- There is a lot of indirect evidence indicating that the amount of a compound that is dissolved in pore water accounts for the principal bioavailable fraction for earthworms. Predictions of accumulation and effects are therefore often based on equilibrium partitioning (EP) theory, assuming that equilibria between soil phases and the organism are established. However, there are several factors that threaten the general applicability of this approach. The actual dissolved concentrations that an organism encounters may be difficult to determine and equilibrium between all phases may not be established. For instance, there are indications that biodegradation of PAHs in soil pore water is sufficiently rapid to lower their local bioavailability for earthworms. A further limitation of EP is that earthworms do not only take up chemicals across their skin but may also take up chemicals through feeding as many species consume large quantities of soil. A model was developed in which uptake via the gut was simulated as well as uptake through the skin. The model was calibrated using existing data for hexachlorobenzene. The results indicate that, although exchange of chemicals across the gut wall is more rapid than across the skin, feeding is not necessarily the dominant exposure route. Furthermore, the results show that large deviations from EP are not expected for earthworms as a result of feeding. However, explicit modelling of the feeding route is necessary in situations where EP does not apply: when the food source of the earthworm is specifically contaminated (e.g. in case of animal pharmaceuticals where residues may end up in the faeces of farm animals). Key words: earthworms, equilibrium partitioning, bioaccumulation, feeding |
