Methanol: Fate and Transport in the Environment - The Toxicology of Methanol - page 19

groundwater compared to the rate of groundwater flow. For nonionic
organic compounds such as methanol, K d values are a function of the
organic carbon fraction of the soil (f oc ) and the organic carbon-based
partition coefficient (K oc (l/kg)). It can be shown that K d

f oc
(Schwarzenbach et al., 1993). Values of f oc are site-dependent and are
typically low in most subsurface environments (in the order of 0.5%)
(Zogorski et al., 1997). K oc values for methanol are low (

¼ K oc

8) (Table 2.3)

relative to values of other gasoline oxygenates ( 12 for MTBE) and

aromatics ( 100 for benzene). Consequently, in a soil/water environ-

ment with typically low soil organic carbon, methanol would be present
primarily in the water phase.
For the listed K oc values, it can be shown that retardation of the rate of
methanol migration due to adsorption would be negligible (Wood et al.,
1990; Zogorski et al., 1997). The retardation factor for methanol and other
short-chain alcohols is usually 1. Thus, dissolvedmethanol wouldmigrate
at the velocity of groundwater. For soils with higher levels of organic
carbon, some retardation can occur. For example, in soils with an organic
carbon fraction > 10% (i.e., for f oc ¼ 0.8, signifying

nearly equivalent concentrations of methanol adsorbed on soil and
dissolved in water. Consequently, methanol would generally travel
through the subsurface at rates significantly higher than constituents in
gasoline, including benzene, toluene, ethylbenzene, or xylenes (BTEX).
However, owing to rapid dissolution and biodegradation, discussed in
further sections, ultimate methanol plume lengths would likely be shorter
than BTEX plumes in the event that simultaneous releases had occurred.

0.1), the K d is

2.2.4 Methanol Dissolution

The dissolution of methanol into groundwater from a methanol product
spill scenario involves dissolution from a pure methanol pool (M100:
pure, 100% methanol). Under this scenario, methanol is likely to leach
quickly from the release area into groundwater. For gasoline releases, the
aqueous concentrations of gasoline hydrocarbons and ether oxygenates in
contact with water can be estimated using Raoult's law (hydrocarbons'
solubility in water multiplied by the mole fraction in the gasoline)
(e.g., maximum theoretical solubility of MTBE ¼ 48,000 ppm 11%