Biomedical Engineering Reference
In-Depth Information
contribute significantly to the loss of methanol from the environment
because of the rapid rates of biodegradation in soil and groundwater and
vaporization from dry soils. Little adsorption is expected for com-
pounds such as methanol because of its polarity, low K
d
value and
miscibility, as discussed previously.
Volatilization from Groundwater and Vaporization from Soil Methanol's
high solubility and low Henry's constant are expected to result in
minimal methanol losses from groundwater because of volatilization
or off-gassing. However, in the soil, methanol would vaporize from the
residual phase or source area under low soil moisture conditions
(ENVIRON, 1996). Vaporization would occur much quicker for
M100 fuels than conventional gasoline because of the high vapor
pressure of methanol. This may result in migration of methanol away
from the source area due to vapor-phase, diffusion-driven transport.
Despite the potential for lateral distribution of methanol in the sub-
surface, analytical soil samples often do not contain detectable levels of
methanol near anM85 spill. This is likely due to themass loss fromvapor
phase microbial degradation of methanol (Beck, 1991).
2.3.1.3 Methanol/BTEX Commingled Plumes
BTEX compounds are
the most toxic group of hydrocarbons in gasoline. Naturally occurring
biodegradation typically mitigates the persistence of these compounds in
the subsurface environment. However, high concentrations of alcohols
and ether oxygenates can potentially impede the biodegradation of BTEX
(Salanitro, 1993). Alcohols would be biodegraded in preference to BTEX
and can therefore consume the oxygen required for BTEXbiodegradation.
Alcohols can also be toxic or inhibitory to subsurface populations of
BTEX-degraders at elevated concentrations (Barker et al., 1990).
Information on the effect of methanol on the biodegradation of BTEX
in the field is available from the Borden site study in Canada. Three
gasolines (M85, 15%MTBE, and base gasoline) were allowed to migrate
in an aquifer (Barker et al., 1990). By Day 476, most of the methanol was
degraded, but BTEX compounds in the methanol plume were at higher
relative concentrations than in the MTBE and nonoxygenated plumes.
These data suggest that methanol may have slowed BTEXbiodegradation
