41
Properties of Pesticides and Contaminants
practices (Majewski et al., 1995). As the fumigant was being injected at
25–30 cm depth, one feld was covered simultaneously with a high-barrier
plastic flm tarp (Figure 3.8). Another feld was fumigated in like manner and
left uncovered, but the furrows made by the injection shanks were bedded
over. The cumulative volatilization losses from the tarped feld were 22% of
the nominal application within the frst 5 days of the experiment. In contrast,
the nontarped feld lost 89% of the nominal application by volatilization in
5 days. These results emphasize the importance of “sealing” a feld with a
tarp to reduce losses of a volatile fumigant, such as MeBr, which is a known
ozone-depleting chemical (see Chapter 8 for more on MeBr).
The TPS method (Majewski et al., 1982; Yates, 2006) requires a circular
feld and wind speed and chemical vapor concentration are measured at one
height (Z inst ) above the feld:
F = (u c)/R
(3.21)
z
i nst
where u (Z inst ) is the average wind speed, c (Z inst ) is the average chemical vapor
concentration, and R inst is the ratio of horizontal to vertical fux at the measurement height (Z inst ). R inst depends on the surface roughness and the radius
of the circular feld but does not depend on the wind speed. The roughness
height is derived from the wind speed profle measurements made prior to
the feld experiment. This method was used in one study to determine the
volatilization losses of the herbicide triallate applied to the surface of fallow
soil (Yates, 2006). This study also compared the TPS method with the IHF
method and found that the two methods gave similar results. A separate
study compared the TPS method with the AG method and found that, while
the two methods gave equivalent results, the TPS method was easier and less
costly to use (Majewski et al., 1989).
For the IHF method (Majewski et al., 1990; Woodrow et al., 1997), a chemical
vapor sampling mast is placed at the downwind edge of the source, and vapor
concentrations are measured at various heights. Wind speed is also measured
at the same sampling heights. At the downwind edge of the source, the chemical vapor plume height will be about 10% of the depth of the source, depending on atmospheric stability. Using the plume height at the downwind edge
and the concentration and wind speed profles, it is possible to calculate the
IHF (F z ):
F = 1/X c
° × u i ˜ z
(3.22)
z
i
where c i and u i are average concentration and wind speed, respectively, at
height z. X is the distance to the upwind edge of the source, and the product c i × u i is integrated over the plume height, z. The setup and equipment
requirement for the IHF method is essentially the same as that for the AG
method, except that IHF does not require the 100:1 fetch that the AG method
needs to give reliable results.
Properties of Pesticides and Contaminants
practices (Majewski et al., 1995). As the fumigant was being injected at
25–30 cm depth, one feld was covered simultaneously with a high-barrier
plastic flm tarp (Figure 3.8). Another feld was fumigated in like manner and
left uncovered, but the furrows made by the injection shanks were bedded
over. The cumulative volatilization losses from the tarped feld were 22% of
the nominal application within the frst 5 days of the experiment. In contrast,
the nontarped feld lost 89% of the nominal application by volatilization in
5 days. These results emphasize the importance of “sealing” a feld with a
tarp to reduce losses of a volatile fumigant, such as MeBr, which is a known
ozone-depleting chemical (see Chapter 8 for more on MeBr).
The TPS method (Majewski et al., 1982; Yates, 2006) requires a circular
feld and wind speed and chemical vapor concentration are measured at one
height (Z inst ) above the feld:
F = (u c)/R
(3.21)
z
i nst
where u (Z inst ) is the average wind speed, c (Z inst ) is the average chemical vapor
concentration, and R inst is the ratio of horizontal to vertical fux at the measurement height (Z inst ). R inst depends on the surface roughness and the radius
of the circular feld but does not depend on the wind speed. The roughness
height is derived from the wind speed profle measurements made prior to
the feld experiment. This method was used in one study to determine the
volatilization losses of the herbicide triallate applied to the surface of fallow
soil (Yates, 2006). This study also compared the TPS method with the IHF
method and found that the two methods gave similar results. A separate
study compared the TPS method with the AG method and found that, while
the two methods gave equivalent results, the TPS method was easier and less
costly to use (Majewski et al., 1989).
For the IHF method (Majewski et al., 1990; Woodrow et al., 1997), a chemical
vapor sampling mast is placed at the downwind edge of the source, and vapor
concentrations are measured at various heights. Wind speed is also measured
at the same sampling heights. At the downwind edge of the source, the chemical vapor plume height will be about 10% of the depth of the source, depending on atmospheric stability. Using the plume height at the downwind edge
and the concentration and wind speed profles, it is possible to calculate the
IHF (F z ):
F = 1/X c
° × u i ˜ z
(3.22)
z
i
where c i and u i are average concentration and wind speed, respectively, at
height z. X is the distance to the upwind edge of the source, and the product c i × u i is integrated over the plume height, z. The setup and equipment
requirement for the IHF method is essentially the same as that for the AG
method, except that IHF does not require the 100:1 fetch that the AG method
needs to give reliable results.