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Aqueous Phase Concentrations Associated with Gasoline
Depending on the fuel's octane (87, 89, or 93), the maximum equilibrium concentrations for BTEX in the groundwater would range from 47 to 72 mg/l; for MTBE the range would be 5,800 to 6,800 mg/l. By comparison in our first transect we saw a high BTEX concentration of 12.5 mg/l and a high MTBE concentration of 5 mg/l.
It is resonable for MTBE and BTEX concentrations to be generally lower downgradient from the source due to the effects of dispersion and some degree of mixing of clean and contaminated water over the length of the sampling point screen (4 ft).
Still, these values are somewhat less than what we might expect. Particularly, the MTBE values should be higher since it is known to be generally recalcitrant with respect to degradation. Also, since the release occurred after the Clean Air Act mandates went into effect, the released fuel would have contained MTBE at 10 to 15 % by volume. Thus it appears that some of the MTBE, at least, is unaccounted for.
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