Passive dosing of soil invertebrates with polycyclic aromatic hydrocarbons: Limited chemical activity explains toxicity cutoff

The partitioning of organic soil pollutants into soil organisms is driven by their chemical activity, which normally does not exceed that of the pure pollutant. Passive dosing with the silicone poly(dimethylsiloxane) (PDMS) was used to initiate and maintain the maximum chemical activity of 10 polycyclic aromatic hydrocarbons (PAHs) in toxicity tests with the springtail Folsomia candida. The test animals could move freely on the PDMS saturated with PAHs, resulting in direct contact and exposure to saturated air. After 7 days, springtail lethality correlated neither with the octanol-water partition coefficients of the PAHs nor with their molecular size, but with their melting point. All low-melting PAHs (T-M <= 110 degrees C) caused 100% lethality, whereas all high-melting PAHs (T-M >= 180 degrees C) caused no significant lethality. The lethality was successfully fitted to one chemical activity response curve for all PAHs tested, with effective chemical activity causing 50% lethality (Ea-50) of 0.058. It was also fitted to the PAH concentration in the PDMS, resulting in an ECPDMS-50 of 8.7 mM. Finally, the combined exposure to anthracene and pyrene was described by the sum of chemical activities causing lethality, in good agreement with the chemical activity-response curve obtained.