Although petroleum hydrocarbons released to the environment typically occur as mixtures, petroleum hydrocarbon remediation guidelines often reflect individual substance toxicity. It is well documented that groups of aliphatic petroleum hydrocarbons act via the same mechanism of action (nonpolar narcosis) and, theoretically, concentration addition mixture toxicity principles apply. To assess this theory, 10 standardized acute and chronic soil invertebrate toxicity tests on a range of organisms (Eisenia fetida, Lumbricus terrestris, Enchytraeus crypticus, Folsomia candida, Oppia nitens, and Hypoaspis aculeifer) were conducted with a refined petroleum hydrocarbon binary mixture. Reference models for concentration addition and independent action were applied to the mixture toxicity data with consideration of synergism, antagonism, and dose level toxicity. Both concentration addition and independent action, without further interactions, provided the best fit with observed response to the mixture. Individual fraction effective concentration values were predicted from optimized, fitted reference models. Concentration addition provided a better estimate than independent action of individual fraction effective concentrations based on comparison with available literature and species trends observed in toxic responses to the mixture. Interspecies differences in standardized laboratory soil invertebrate species responses to petroleum hydrocarbon-contaminated soil was reflected in unique traits. Diets that included soil, large body size, permeable cuticle, low lipid content, lack of ability to molt, and no maternal transfer were traits linked to a sensitive survival response to petroleum hydrocarbon-contaminated soil in laboratory tests. Traits linked to sensitive reproduction response in organisms tested were long life span and small clutch size. By deriving single-fraction toxicity endpoints considerate of mixtures, we can reduce the resources and time required to conduct site-specific risk assessments for the protection of a soil organism's exposure pathway. Environ Toxicol Chem 2018;37:2222-2234. © 2018 SETAC.
