Evaluating the sensitivity of a chronic plant bioassay relative to an independently derived predicted no effect thresholds to support risk assessment of very hydrophobic organic chemicals

Abstract

Environmental risk assessments require high quality toxicity data to establish protective thresholds. The chronic effects of very hydrophobic organic compounds (VHOCs) in soils are often difficult to determine because multiple processes (e.g. sorption, volatilization, biodegradation) can complicate the interpretation of results. We have developed a standardized soil dosing and aging procedure for assessing bioavailability of high logK_ow VHOCs in a synthetic soil surrogate, and then used it to evaluate the toxicity of high logK_ow VHOCs across a range of test substance concentrations and soil organic carbon content. The soil preparation protocol resulted in relatively stable freely dissolved concentrations of test substance compared to bulk soil concentrations with some losses likely due to volatility and biodegradation. This dosing method wasused in a chronic terrestrial plant toxicity bioassay to evaluate the potential toxicity of VHOCs on complex reproductive endpoints like inflorescence and seed bud formation. Testing included common hydrocarbons and three very hydrophobic lubricant substances (logKow > 10). The toxicity data were used to evaluate existing predicted no effect concentrations (PNEC) that had originally been derived without these higher order chronic plant endpoints. The initial exposure concentrations were set at the independently-derived PNECs to provide an independent validation of the PNEC framework. This evaluation was performed to expand the domain of applicability of the PNEC to VHOCs and for the chronic terrestrial plant endpoints. We saw no effects on plant biomass or inflorescence production at these low exposure concentrations, demonstrating that the established PNEC is protective of long term plant health. The results of the present study confirm that the new dosing method is fit for purpose, and that the existing PNEC framework can be extended to chronic plant endpoints for VHOCs.