Biomedical Engineering Reference
In-Depth Information
by volume in a given gasoline
5280 ppm) (Poulsen et al., 1992).
However, because methanol is completely miscible in water, this method
is not applicable, and the concentration must be estimated based on
presumed mixing conditions using a methanol/groundwater mixing ratio
and a dilution factor.
Assuming that subsurface mixing ratios are between 10 and 100, the
maximum range of methanol concentrations would be 0.9-9.1% by
volume (7000-70,000 ppm by mass) for an M100 release. However,
these high concentrations would only occur in the immediate vicinity of
pure methanol releases. Much lower concentrations are anticipated
further from the source due to methanol plume dilution and dispersion.
Alternatively, the concentration of methanol in groundwater can be
approximated by taking the ratio of the release flow rate and the ground-
water velocity and assuming a source areawidth and vertical mixing ratio.
For example, if methanol is released from an underground storage tank at
0.5 gpm, groundwater is moving at 1 ft/day, the source areawidth is 1m,
and the vertical mixing ratio is 10-100, the resulting methanol concen-
tration in the groundwater would be between 70 and 700 ppm. These
concentrations would be much larger in the immediate vicinity of a
catastrophic underground storage tank release (i.e., complete tank failure)
or barge release (to surface water). However, in most cases, the infinite
solubility of methanol would cause methanol concentrations to rapidly
decrease as a result of mixing, dispersion, and diffusion.
Poulsen et al. (1992) evaluated the rate of dissolution of methanol from
an M85 fuel (85% methanol and 15% unleaded gasoline) into water.
Poulsen et al. (1992) experimentally determined that
ΒΌ
99% of the
methanol would leach into the water phase in the time required for three
volumes of water, each equal to the initial gasoline volume, tomove past the
gasoline. They concluded that this would result in a short discrete plume.
The rate of methanol dissolution into the subsurface following a
release is similar to the rate of ethanol dissolution because methanol and
ethanol have similar properties. Heermann and Powers (1997, 1998)
studied the partitioning behavior of ethanol and BTEX compounds from
an ethanol-blended gasoline. According to Heermann (1998, personal
communication), the aqueous concentration of ethanol due to dissolu-
tion of ethanol-blended gasoline is dependent on the concentration of
