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ity of 1.8g/L. Most gasoline constituents have solubilities of less than 1 g/L, and some dissolve to
only a few mg/L. This is the key characteristic that causes so much of the mass of an MTBE release
to dissolve in groundwater. The higher solubility is associated with the greater polarity of
the compound.
The Henry’s Law constant is an indicator of the equilibrium distribution of a compound be-
tween water and air. When compared to benzene, MTBE tends to partition strongly into the water
phase. If the same units are selected for the air and water concentrations, then Henry’s Law
constant is dimensionless. The value of the dimensionless Henry’s Law constant for MTBE is
0.0216 at 25o
C (Robbins et al., 1993). In contrast, the dimensionless Henry’s Law constant for
benzene is 0.22 at 25o
C, which indicates that it transfers easily from water and can be removed by
aeration. Compared to benzene, MTBE tends to stay in the water phase, which explains why
MTBE is somewhat difficult to remove from water by aeration.
The organic carbon water partition coefficient (Koc
) is a reflection of the compound’s
tendency to sorb to the organic carbon matrix within soil systems. The organic carbon sorption will
retard the migration of the compound. With a value of Koc
equal to 11 and an octanol water parti-
tion coefficient (Kow) of 17.4 (Zhang et al., 1998a; Squillace et al., 1997), MTBE is less retarded
than other gasoline constituents.
Compared to other gasoline constituents, the physicochemical properties of MTBE and
many other oxygenates present significant issues when considering treatment options and the fate
and transport of these pollutants in the environment. Given the high water solubility, MTBE is quite
mobile in the environment. It partitions weakly to the organic fraction in soils, sediments, and
suspended particles, preferentially remaining in the aqueous phase. It is expected to move essen-
tially at the same rate as groundwater flow, with practically no retardation due to sorption.
Ethers are a class of compounds that are characteristically unreactive over a wide range of
industrial and laboratory conditions, so it is unlikely that MTBE will be degraded rapidly in theaquatic environment (Church et al., 1997). MTBE is a persistent molecule in the environment for
several reasons: (1) the ether bond is stable and requires acidic conditions to cleave it; (2) the bulky
tert-butyl group does not allow easy access to the ether linkage; (3) MTBE is not a naturally
occurring hydrocarbon unlike most oil and gasoline constituents; and (4) it has only been in the
environment for a relatively short time, so there has been little selection for indigenous microbes to
transform MTBE. The biodegradability of MTBE is generally presumed to be significantly less than
the degradability of other gasoline constituents. Initial studies indicate that biodegradation of MTBE
in the environment is slow (Borden et al., 1997; Mormile et al., 1994; Suflita and Mormile, 1993).
A field study by LeBrun (1993) utilized a contaminant mass flux approach to estimate rates of
intrinsic bioremediation for MTBE and BTEX at a gasoline-contaminated shallow aquifer in
Sampson County, N.C. His results suggested that MTBE degradation was occurring near the
source area at a rate of 0.13%/day. Since the contaminant plumes appear to have been degrading
under a mixture of aerobic and denitrifying conditions, it is unclear which mechanism would have
contributed to MTBE decay.
However, a substantial record of MTBE biodegradation in both laboratory and full-scale
treatment operations has now been accumulated. At least a few bacterial species are able to use