Comparative and combined toxicities of toluene and methyl tert-butyl ether to an Asian earthworm Perionyx excavatus

An earthworm assay was used to assess the toxicity of two main gasoline components. Ecotoxicities of toluene and methyl tert-butyl ether (MTBE) were compared for Asian earthworm Perionyx excavatus separately and in combination. Sum of toxic unit (TU) at 50% mortality for the mixture (LC50mix) was estimated from the dose (TU-based)-response relationships by the Trimmed Spearman-Karber method. Toluene was shown to be about five and two times more toxic to P. excavatus than MTBE in filter paper contact test and natural soil tests, respectively. This is primarily due to the adsorption of toluene to soil particles, leading to less bioavailability of toluene to the earthworm in soil. The combined effect of toluene and MTBE was investigated using toxic unit model, and it was found to be almost additive and synergistic responses to P. excavatus in filter paper contact test and soil tests, respectively. The combined effect of toluene and MTBE in filter paper contact test was not consistent with the results in soil toxicity tests. This phenomenon may be associated with the interaction of soil salts with pollutants. Treatment with toluene and MTBE also affected the behavior and morphology of P. excavatus. It indicates that the VOCs induce metabolic and functional damages in earthworms. Combined effects of gasoline components should be taken into account to soil risk assessment.