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12/1995 20.44 9163519357 ANNE M FAIR PAGE 041 U l EPA allows the blending of MTBE into gasoline up to about 11%by volume The exact proportions of MTBE in a particular gasoline depend on the composition of the cnide,the octane rating of the istillate. and the required octane rating of the final blend. MTBE is usually blended at between 2 and 7%. MTBE is used in both unleaded and "regular" gasol:nes Usually the highest proportions are to be found in unleaded premium. (ARCO. personal communication.) Among other claims made about MTBE are: 1) that it has a blending octane value greater than that of toluene.reformate or alkylate; 2) that it is compatible with all types of automotive and tank liner materials; 3) that it docs not have a phase separation problem in the presence of water (which is a problem unth ethanol and methanol): and 4) that its use in gasoline reduces carbon monoxide and hydrocarbon emissions in most cars (from ARCO's advertising pamphlets). Transport and Fate of 1WTIIE in the Environment MTBE is soluble in water at 4.3% (ARCO data). This compares to the relative insolubtWv of other major components of gasoline,as summarized in Table 1. Table 1: Solubility of some major gasoline components t binary equilibrium solubility in water at 25 to % relative to benzene benzene 1.780 0.18 1 toluene 515 0.05 0.3 m xylene 170 0.02 0.1 MnE 43,000 4.3 24 (Data from API. 1995.Table 2-1 for BTX.That source gives a range of values for each component Data for MTBE from ARCO factsheets.) Though it is important to realize that the equilibrium solubilities listed above are rarely reached in nature, the significance of the figures lies in their ability to predict how MTBE behaves as a ground water PI g85 mblrelative to other components of gasoline. of which benzene is the most soluble(A perhaps MTBE is only used as presence IyRBve ofE asol�und watcrecan assume significs the nearby prcts�nce of around rcfinutes, the pros gasoline. ' When gasoline leaks ground water. it contaminates the unsaturated zone amen a ata table with free product in the vic cr inity of the leak. Recharge to ground water by Q percolates through this product contamination zone, dissolving components eagasoline infer ! approximate proportion to their solubilities. Although wivll be considerable know of no y enhanced over that of I that the concentration of urmE in the recharge the other components of gasoline for the following two reasons: other ollents of Fust, MTBE is at least an orderof umagnitude l Iof the pine product, le than may be one of gasoline. Second. at a concentrationp the largest components of the spilled gasoline;