Fumigants
127
Most nations have agreed to phase out ozone depleters like MeBr per the
Montreal Protocol agreements. However, the phaseout has exemptions
which the United States has applied for since 2004. The exemptions are
based on the idea that fumigants are vital agents in the production of many
food crops, particularly high-value crops such as strawberries and grapes,
which are susceptible to nematodes and other soil-borne pests. They are
also used—even required—before and during shipment of fruits, grains,
and spices for export (Phillips et al., 2012; Walse, USDA ARS). These exemptions contend there are no suitable alternatives, thus some MeBr use continues even though it is still subject to phase out (see Table 8.1). The use
has declined over time partly due to threat of phaseout and partly because
growers have implemented alternatives to some MeBr uses. These alternatives include chloropicrin and 1,3-D. Nonchemical alternatives have also
been developed such as soil solarization and use of nematode-resistant
crop plants.
MeBr is predominantly a naturally occurring compound; anthropogenic
(synthetic) sources represent only around 25% (±10%) of total emissions.
Furthermore, oceans and soil were identifed as sinks, which signifcantly
decreased its ozone depletion potential from 0.7 to about 0.45 and overall lifetime from 2.0 to between 0.8 and 1.0 years (Honaganahalli and Seiber, 1996).
(Note that CO 2 has an ozone depletion of 1.0.) Further, MeBr has tested mostly
negative as a carcinogen. Stringent fumigation rules, at least in the United
States, have been effective in reducing exposure. In light of this information,
MeBr may be a better fumigant than some of its replacements. Instead of an
outright ban, better management practices may be suffcient for its continued
use as a fumigant (Honaganahalli and Seiber, 1996).
Recent controversy over the potential role of MeBr in damaging the ozone
layer has spurred interest in increasing our understanding of the transformation and movement of this fumigant after it is applied to soil. Our
research indicates MeBr is rapidly volatilized from fumigated soil (usually
within the frst 24 hours) and volatility signifcantly increases with temperature (35°C > 25°C > 15°C) and moisture (0.03 bar ≥ 0.3 bar > 1 bar > 3 bar).
Degradation of MeBr, measured by production of bromide ion (Br - ), was
also directly related to temperature and moisture. Undisturbed soil column studies indicated that MeBr rapidly volatilized (>50% of the MeBr fux
occurred in 48 hours) but did not leach into subsurface soil in that amount
of time. Residual MeBr was degraded eventually in the soil column, evident by measurable concentrations of Br - in the leachate water. In feld studies, MeBr also volatilized rapidly from soil, but a signifcant portion of the
MeBr was degraded (30% after 2 days). These studies provide pertinent
information for assessing the fate of MeBr in soil, which should lead to
more informed decisions regulating its use. (Reprinted with permission
from Anderson et al. (1996). Fate of Methyl Bromide in Fumigated Soils.
In Fumigants, (American Chemical Society), pp. 42–52.). Copyright (1996)
American Chemical Society.)
127
Most nations have agreed to phase out ozone depleters like MeBr per the
Montreal Protocol agreements. However, the phaseout has exemptions
which the United States has applied for since 2004. The exemptions are
based on the idea that fumigants are vital agents in the production of many
food crops, particularly high-value crops such as strawberries and grapes,
which are susceptible to nematodes and other soil-borne pests. They are
also used—even required—before and during shipment of fruits, grains,
and spices for export (Phillips et al., 2012; Walse, USDA ARS). These exemptions contend there are no suitable alternatives, thus some MeBr use continues even though it is still subject to phase out (see Table 8.1). The use
has declined over time partly due to threat of phaseout and partly because
growers have implemented alternatives to some MeBr uses. These alternatives include chloropicrin and 1,3-D. Nonchemical alternatives have also
been developed such as soil solarization and use of nematode-resistant
crop plants.
MeBr is predominantly a naturally occurring compound; anthropogenic
(synthetic) sources represent only around 25% (±10%) of total emissions.
Furthermore, oceans and soil were identifed as sinks, which signifcantly
decreased its ozone depletion potential from 0.7 to about 0.45 and overall lifetime from 2.0 to between 0.8 and 1.0 years (Honaganahalli and Seiber, 1996).
(Note that CO 2 has an ozone depletion of 1.0.) Further, MeBr has tested mostly
negative as a carcinogen. Stringent fumigation rules, at least in the United
States, have been effective in reducing exposure. In light of this information,
MeBr may be a better fumigant than some of its replacements. Instead of an
outright ban, better management practices may be suffcient for its continued
use as a fumigant (Honaganahalli and Seiber, 1996).
Recent controversy over the potential role of MeBr in damaging the ozone
layer has spurred interest in increasing our understanding of the transformation and movement of this fumigant after it is applied to soil. Our
research indicates MeBr is rapidly volatilized from fumigated soil (usually
within the frst 24 hours) and volatility signifcantly increases with temperature (35°C > 25°C > 15°C) and moisture (0.03 bar ≥ 0.3 bar > 1 bar > 3 bar).
Degradation of MeBr, measured by production of bromide ion (Br - ), was
also directly related to temperature and moisture. Undisturbed soil column studies indicated that MeBr rapidly volatilized (>50% of the MeBr fux
occurred in 48 hours) but did not leach into subsurface soil in that amount
of time. Residual MeBr was degraded eventually in the soil column, evident by measurable concentrations of Br - in the leachate water. In feld studies, MeBr also volatilized rapidly from soil, but a signifcant portion of the
MeBr was degraded (30% after 2 days). These studies provide pertinent
information for assessing the fate of MeBr in soil, which should lead to
more informed decisions regulating its use. (Reprinted with permission
from Anderson et al. (1996). Fate of Methyl Bromide in Fumigated Soils.
In Fumigants, (American Chemical Society), pp. 42–52.). Copyright (1996)
American Chemical Society.)