Geoscience Reference
In-Depth Information
The mode of entry to the subsurface environment affects partitioning among
subsurface components. Considering that the composition of gasoline depends on
the nature of the emission process, Foster et al. (
2005
) developed a model scenario
where a known amount of gasoline emission was divided equally among air (gas),
water, and soil. The composition of gasoline in the air phase was assumed to be in
the form of a fugitive vapor from liquid gasoline, while emissions to soil or to
water were considered as liquid gasoline. Figure
8.2
exhibits a four by four set of
charts displaying the relative gasoline composition by mass in air, water, soil, and
sediment as a result of three modes of entry and the total simultaneous emission.
For each mode of entry, the percentage of gasoline by mass and the concentration
in each of the 24 compartments was calculated and the contribution of the mode of
entry estimated.
When the emission of gasoline is mainly into the air phase, more than 99 % of
the contaminant remains in the air. In this case, the gasoline emission consists
predominantly of alkanes (70 % of the total gasoline composition) with a lower
number of carbon atoms. When gasoline emission is mainly to water, 85 %
remains in the aqueous phase, including groups 5, 6, 7, 19, and 21 (see Fig.
8.1
),
about 8 % partitions into sediment, and less than 1 % into soil. The mode of entry
only into soil leads to retention of about 91 %. From Fig.
8.2
, we see that
simultaneous entry of a total emission of gasoline leads to a partitioning of 52 % in
soil, 31 % in water, 14 % in air, and 3 % in sediments. This scenario and calcu-
lation provide a representative overview of contaminant partitioning among the
gaseous, aqueous, and solid phases, as affected by the properties of the contami-
nants, the surrounding phase, and their mode of entry into the environment.
8.2 Contaminant Volatilization
Contaminant volatilization from subsurface solid and aqueous phases may lead, on
the one hand, to pollution of the atmosphere and, on the other hand, to contami-
nation (by vapor transport) of the vadose zone and groundwater. Potential vola-
tility of a contaminant is related to its inherent vapor pressure, but actual
vaporization rates depend on the environmental conditions and other factors that
control behavior of chemicals at the solid-gas-water interface. For surface
deposits, the actual rate of loss, or the proportionality constant relating vapor
pressure to volatilization rates, depends on external conditions (such as turbulence,
surface roughness, and wind speed) that affect movement away from the evapo-
rating surface. Close to the evaporating surface, there is relatively little movement
of air and the vaporized substance is transported from the surface through the
stagnant air layer only by molecular diffusion. The rate of contaminant volatili-
zation from the subsurface is a function of the equilibrium distribution between the
gas, water, and solid phases, as related to vapor pressure solubility and adsorption,
as well as of the rate of contaminant movement to the soil surface.
