Decreased toxicity to terrestrial plants associated with a mixture of methyl tert-butyl ether and its metabolite tert-butyl alcohol.

The influence of the main fuel oxygenate methyl tert-butyl ether (MTBE) and its key metabolite, tert-butyl alcohol (TBA), on the growth of a plant seedling was studied separately and in combination. The test plants were mung bean (Phaseolus radiatus), cucumber (Cucumis sativus), wheat (Triticum aestivum), sorghum (Sorghum bicolor), kale (Brassica alboglabra), Chinese cabbage (Brassica campestris), and sweet corn (Zea mays). The growth of all the plants was adversely affected by TBA and MTBE. The 5-d median effective concentration (EC50) for the plants exposed to MTBE and TBA were in the range of 680 to 1,000 mg MTBE/kg soil (dry wt) and 1,200 to 3,500 mg TBA/kg soil (dry wt), respectively. The relative order of the sensitivity rankings is almost the same for MTBE and TBA. Methyl tert-butyl ether is more toxic than TBA to most of the test species. Based on the EC50 values, MTBE is approximately 1.5 to 3 times more potent than TBA. The sum of the toxic unit (TU) at 50% inhibition of the mixture (EC50mix) was calculated from the dose (TU-based)-response relationships using the trimmed Spearman-Karber method. The combined effect of MTBE + TBA on the plant growth was less than additive because the EC50mix values were greater than I TU. This phenomenon may be due to the competition of MTBE and TBA in terms of their intake by plants. The combined effects of MTBE and TBA should be taken into account to assess their risk in gasoline-contaminated sites.