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vapor densities, even when the pesticide concentration in the moist soil was high
enough to yield vapor densities approaching those of the pure compound. These
results explain why reduction in pesticide volatilization in dry soils was observed
over many years.
Wolters et al. (
2003
) observed that volatilization kinetics of the fungicide
fenpropimorph express a clear correlation between volatilization rates and soil
moisture content. Volatilization rates reached a maximum 24 h after application
under moist conditions and decreased with the decrease in soil moisture over
following days.
8.2.3 Mixtures of Organic Contaminants
Volatilization from mixtures of organic contaminants brings about changes in both
the physical and the chemical properties of the residual liquid. We consider data on
kerosene volatilization, as summarized in Yaron et al. (
1998
). Kerosene is an
industrial petroleum product composed of more than 100 hydrocarbons, which
may become a subsurface contaminant.
The differential volatilization of neat kerosene components from a liquid phase,
directly into the atmosphere during volatilization up to 50 % (w/w), is presented in
Fig.
8.8
. Ten kerosene components were selected, and their composition was
depicted as a function of gas chromatograph peak size (%), which is linearly
related to their concentration. It may be seen that the lighter fractions evaporate at
the beginning of the volatilization process. Increasing evaporation causes addi-
tional components to volatilize, which leads to a relative increase in the heavier
fractions of kerosene in the remaining liquid.
In the subsurface, kerosene volatilization is controlled by the physical and
chemical properties of the solid phase and by the water content. Porosity is a major
factor in defining the volatilization process. Galin et al. (
1990
) reported an
experiment where neat kerosene at the saturation retention value was recovered
from coarse, medium, and fine sands after 1, 5, and 14 days of incubation. The
porosity of the sands decreased from coarse to fine. Figure
8.9
presents gas
chromatographs obtained after kerosene volatilization. Note the loss of the more
volatile hydrocarbons by evaporation in all sands 14 days after application and the
lack of resemblance to the original kerosene. It is clear that the pore size of the
sands affected the chemical composition of the remaining kerosene. For example,
the C
9
-C
12
fractions disappeared completely 14 days after their application,
except for the saturated fine sand case, where 5 % of the initially applied C
12
remained.
The effect of aggregation of the subsurface solid phase on kerosene volatili-
zation was studied by Fine and Yaron (
1993
), who compared the rate of aggre-
gation in two size fractions of a vertisol soil: the \1 mm fraction and 2 mm
aggregates. The total porosity of these two fractions was similar (53 and 55 % of
the total volume, respectively). Differences in aggregation are reflected in the air
