Derivation of Soil Ecotoxicity Guidelines for Petroleum Hydrocarbons Using Target Lipid and Equilibrium Partitioning Models

Hydrocarbon contaminated soils are commonly encountered in Urban settings and the resulting ecotoxicity is difficult to evaluate due to the complex chemical composition of sources. An extension of the hydrocarbon block method is used to derive soil predicted no-effect concentrations (PNECs) for use in environmental risk assessments. This method uses a database of physicochemical properties for nearly 1500 individual, representative hydrocarbons to define larger hydrocarbon blocks, or fractions, in terms of individual compounds as a function of chemical class and boiling point characterization. Soil PNECs are calculated using a combination of Equilibrium Partitioning Model with the Target Lipid Model (TLM). The TLM provides a toxicity database that is used to estimate PNECs for each structure assigned to a hydrocarbon block. This framework allows soil guidelines to be established for individual hydrocarbons or as Total Petroleum Hydrocarbon (TPH) fractions that are characteristic of crude oils or refined petroleum products (e.g., gasoline, diesel, etc). An important modification to the TLM is the introduction of a lipid membrane-water partition cut-off for hydrocarbon constituents with log(K_OW) > 6 that accounts for processes that reduce the bioavailability of very hydrophobic compounds. The PNECs calculated with this model are compared to measured soil toxicity data for terrestrial plants, microbes and invertebrates. This approach offers improvement for environmental site assessments by allowing the risks posed by different TPH composition in contaminated soils to be taken into proper consideration.