Application of the target lipid and equilibrium partitioning models to non-polar organic chemicals in soils and sediments

The Target Lipid Model (TLM) provides a framework for deriving predicted no effect concentrations (PNEC) for non polar organic chemicals to organisms in the environment. This approach has been used to perform environmental risk assessment of individual hydrocarbons as well as complex petroleum substances. The TLM is based primarily on data for aquatic test organisms and this work evaluates the potential for extending the TLM to soil and sediment using Equilibrium Partitioning (EqP) theory.

Report no. 1/14: Literature data for other nonpolar organics were compiled for acute and chronic exposures to invertebrates in soils and sediments. New data were generated according to OECD guidelines (CONCAWE, 2011 and 2012) to evaluate soil and sediment dwelling organisms and to test potential toxicity cut-offs for high log Kow compounds. The default TLM was applied to these data using EqP to develop critical target lipid body burdens (CTLBB) including associated uncertainty in the model application.

Comparison of the CTLBBs for soil and sediment species to CTLBBs from the larger TLM database for aquatic organisms showed little difference in the relative sensitivity between these two groups of species within the uncertainty of the model and experimental data. Furthermore, the acute to chronic ratios (ACRs) for soil and sediment tests were within the range of ACRs for aquatic organisms exposed to nonpolar organic chemicals.

The TLM-derived PNEC applied to these data, also, demonstrated sufficient level of protection approximately 95% of data above PNEC, even for chemicals up to log Kow 6. For chemicals with log Kow >6 an increasing incidence of no observed toxicity consistent with the dataset for aquatic organisms was observed. The duration of the pre-equilibration step was important for some chemicals. For example, toxicity was observed for these chemicals following short pre-equilibration times (whereas no toxicity was observed for spiked soils that had been aged up to 7 weeks prior to exposure. In conclusion, the work shows that the TLM can be extended to the soil and sediment compartments using the EqP for the purposes of a tier 1 risk assessment.