Modelling non-equilibrium concentrations of microcontaminants in organisms: comparative kinetics as a function of species size and octanol-water partitioning.

From our experience, risk assessment for environmental management and research purposes is in need of models that apply to many of the species we want to protect from many of the microcontaminants released. The traditional one-compartment model serves as such a tool during interpretation and extrapolation of information on concentration kinetics. Unfortunately, its non-steady parameters are specific for a combination of a compound and a species. So, one must seriously face the prospect that their values will never be measured for most compounds and species due to experimental, ethical and financial constraints. It was therefore considered worthwhile to relate the main non-steady state parameter, viz. the outflow (elimination, clearance, depuration) rate, to common characteristics of compounds and species. The outflow rate (kout) for persistent organic microcontaminants was correlated to the octanol-water partition ratio of the compound (Kow) and the size of the species (z). The regressions for aquatic invertebrates, fish and warm-blooded animals were kout = (1/(410(-3)Kow + 710(-8)) + 510(-3))z-0.36 (n = 53, r2 = 0.45), kout = (1/(410(-4)Kow + 5) + 410(-3))z-0.19 (n = 140, r2 = 0.68) and kout = (1/(310(-4)Kow + 210(-5)) + 810(-3)) z-0.86 (n = 51, r2 = 0.48) respectively. The correlation was less strong if Kow or z were omitted. In addition to the minimum loss rate for persistent compounds, one may distinguish an excess outflow rate (mainly caused by biotransformation) for less persistent organic microcontaminants. The order of magnitude difference is explored and ways to refine these estimations are discussed briefly. Outflow rates for cadmium and mercury are linked to species size with the same type of function. The internal consistency of the model was verified by calculating inflow rates from calibrated outflow rates and comparing these to independent measurements. Moreover, the constants in the regressions are explained physically and their value is compared with those obtained in ecology for consumption, production and respiration. The exponent that scales these rates to the species size is similar to the regressions for outflow rates obtained here. The model allows estimations for fairly unknown substances or species and it is thought to help refining risk evaluations without extensive experimental or desk studies. As this paper shows that joint application of chemical (Kow) and ecological (z) information yields more accurate estimations, this study contributes to the often advocated integration of both disciplines in ecotoxicology.