Toxicokinetics in aquatic systems: Model comparisons and use in hazard assessment

Abstract

-Toxicokinetic models are not constrained by assumptions of equilibrium as are thermodynamic (equilibrium-partitioning) models and are more accurate predictors of toxicant accumulation for non-steady-state exposures and multiple uptake routes. Toxicokinetic models -compartmentbased models, physiological-based models, and energetics-based models-are reviewed and the different mathematical formalisms compared. Additionally, the residue-based toxicity approach is reviewed. Coupling toxicokinetic models with tissue concentrations at which toxicity occurs offers a direct link between exposure and hazard. Basing hazard on tissue rather than environmental concentrations avoids the errors associated with accommodating multiple sources, pulsed exposures, and non-steady-state accumulation. Keywords-Kinetic models Bioaccumulation Tissue residue effects Sediment contaminationHazard assessment INTRODUCTlONAssessment and prediction of toxicant effects on aquatic organisms require evaluation of the extent of organism exposure. Exposure assessment establishes the relationship between environmental toxicant concentrations and organism accumulation while accounting for environmental and biological factors that modify exposure. If the relationships between the amount of toxicant accumulated and the resulting effects are known, then the hazard for a particular exposure regime can be established.Aquatic exposure assessments and predictions have employed mainly steady-state and equilibriumpartitioning models. Early efforts, using simple kinetic models, were designed to provide estimates of steady-state accumulation from water exposures [1,2]. These steady-state estimates were then utilized in hazard assessments based on thermodynamic limits (chemical equilibrium). Such models have been employed with good success for evaluation of general conditions, describing toxicant distribution among ecosystem components and *To whom correspondence may be addressed.identifying components dominating toxicant mass balance. This approach has been best refined using the fugacity concept and applied to describe the importance of sediment as a toxicant source [3] and toxicant distributions within ecosystems [4,5].Although there is a continued focus on equilibrium-partitioning models within regulatory agencies, it is clear that the environment is complex and variable. Therefore, to obtain more accurate predictions and assessments, kinetic models are needed to predict non-steady-state, nonequilibrium accumulation from temporally and spatially varying exposures when the simplifying assumptions of the equilibrium-partitioning models are inappropriate, for example, when multiple sources contribute significantly to accumulation.Kinetic models have been used successfully in pharmacology for decades. Such models permit prediction of the onset of drug action and allow the monitoring of drug clearance and termination of effects. Further, these kinetic models describe changes in tissue concentrations resulting from absorption, distribution, metabolism, a...

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