Over the last 40-50 years methyl bromide (CH₃Br) has been used throughout the world to sterilize soils in preparation for planting various high-cash-value fruit and vegetable crops. Highly toxic, CH₃Br is very effective in controlling a variety of soil-borne pests, such as nematodes, weeds and fungi. CH₃Br has been an important component of agricultural systems in the U.S. and its phase-out is expected to cause financial hardship to agricultural producers. Recent economic assessments estimate that more than $1.5 billion in annual lost production would occur in the United States alone [NAPIAP, 1993; Ferguson and Padula, 1994].

In most commercial operations, CH₃Br is applied from a tractor pulling two or more metal shanks that cut into the soil. CH₃Br is injected into the soil at approximately 25 cm depth from nozzles on the backside of each shank. Simultaneously, the tractor lays down a 3.5 m wide sheet of 0.025 mm thick high-density polyethylene (HDPE) plastic film; burying one side and gluing the other side to the previous plastic sheet. This creates a series of panels down the field and a continuous cover over the field. Large amounts of CH₃Br are applied at rates ranging from 200 to 400 kg/ha.

Emission of CH₃Br into the atmosphere is affected to a large degree by the properties of the soil, ambient environmental conditions, application methods, and properties of the plastic film used to seal the surface. Recent research has shown that the traditional HDPE film is largely ineffective in containing CH₃Br in soil. This can be seen from Figure 1, where the fraction of CH₃Br lost to the atmosphere is presented as affected by various fumigation practices (e.g., shallow vs. deep injection, cover vs. no cover, HDPE vs. Hytibar, dry soil vs. sealing the soil surface with water).

Figure 1. Comparison of total emission loss as affected by various fumigation practices. Shallow (25-30cm) and deep (50-70cm) indicate the injection depth.

This figure includes all recent experiments on CH₃Br emissions into the atmosphere [Yagi et al., 1993, 1995; Majewski et al., 1995; Yates et al., 1996, 1997; Williams et al., 1997; Wang et al., 1997]. It is clear that HDPE's effectiveness in controlling emissions is similar to that of bare soil, deep injection or applying water to seal the soil surface. The use of a virtually impermeable film (VIF) such as Hytibar, however, shows a dramatic reduction in the emission rate.

Hytibar is many times less permeable than HDPE, as shown by the film's mass transfer coefficient (Table 1). The mass transfer coefficient is a property of a film that characterizes the ease with which a chemical passes through the film. For standard HDPE, the mass transfer coefficient is 0.35 cm h-1 while that for Hytibar is almost three orders of magnitude lower.

Table 1. Mass transfer coefficients (cm h-1) for CH3Br and chloropicrin diffusing through four plastics at 20°C. The data were obtained using static permeability cells [Papiernik et al., 1999]. Film permeability (cm2 h-1) can be obtained by multiplying the mass transfer coefficient by the film thickness.

§ - preliminary value.
^¶ - not measurable after 40 days.

If the film plays an integral role in controlling the emission process, one would expect that the ratio of the observed emissions from a field covered with HDPE to one covered with VIF would be similar to the ratio of the permeability of HDPE to VIF. For example, the ratio of the film permeability (HDPE/Hytibar) has been estimated to be from 200 [Wang et al., 1998] to 900 (Table 1). These values were obtained in an ideal system where there is no degradation and the film remains in ideal condition. In the field, however, the film is subjected to a harsh environment and one would expect the field-scale permeability to be affected by stretching, tears, holes and the seams between plastic sheets. Therefore, the effective permeability would be less than this ideal case. In a recent experiment using Hyti-bar conducted in the field, the ratio of the total emissions from plots covered with HDPE to those covered with Hytibar was approximately 33 [Yates et al., 1998]. This result suggests that total emissions can be reduced by an order of magnitude by using VIF.

The main disadvantage of using VIFs alone is the need to keep the soil covered for several weeks to avoid the release of fumigant when the tarp is removed. Soil degradation is highly variable and depends primarily on the soil water content and soil organic matter content. However, CH₃Br can be degraded in soil by hydrolysis, with a half life of approximately 50 d, or by reaction with nucleophilic functional groups (e.g., –NH2, –NH, –SH, –OH) in soil organic matter or soil amendments. The rate of nucleophilic substitution is highly variable and depends on the quantity and type of functional groups available in soils. In loamy soils with greater than 1% organic matter, observed degradation half-lives range from 4 to 21 days [Arvieu, 1983; Gan et al., 1998a].

The addition of soil amendments (e.g., organic matter or thiosulfate materials) can enhance the degradation of CH₃Br. Recent research has demonstrated that significant reductions in emissions of CH₃Br and other soil fumigants can be obtained by simply adding organic material or a thiosulfate fertilizer to the soil surface [Gan et al., 1998a,b]. Shown in Figure 2 is the fraction of fumigant lost from soil after the soil has been amended with either organic material or ammonium thiosulfate.

The addition of organic material can help reduce CH₃Br emission by as much as 25%. Spraying a thiosulfate amendment onto the soil surface after fumigation can reduce emissions by an order of magnitude, provided a sufficient quantity of thiosulfate is applied. Ammonium thiosulfate is a commonly used agricultural fertilizer and would add only a small cost to soil fumigation. In addition, a fertilizer amendment might be needed anyway to help add nutrients for the growing season.

Therefore, CH₃Br emissions can be significantly reduced by improving containment, or enhancing soil degradation, or both. Since various transport processes also affect the total degradation in soil, CH₃Br emissions can be managed, provided the chemical remains in the soil for a sufficient amount of time. Enhancing soil degradation by adding nucleophilic material may provide added benefit to the soil (e.g., nutrients or soil conditioners) and could increase the degradation rate so that VIF would not have to remain on the soil surface for excessively long periods. If market competition reduces the price of VIF, the fumigation cost should not increase appreciably. Also, since VIFs maintain higher fumigant concentrations for longer times, the fumigation cost may even be reduced due to a lower application rate of CH₃Br needed for pathogen control.

As the CH₃Br phase-out date approaches, some questions remain whether restricting CH₃Br use will have a significant effect on stratospheric ozone levels [Hona-ganahalli and Seiber, 1997]. Further, it appears that methodology exists that would enable CH₃Br emissions from fumigated soils to be reduced by at least one order of magnitude. This would reduce the global CH₃Br contribution from agricultural use to less than 1% [Yates et al., 1998] of the worldwide sources.

The question that seems to have been overlooked in deriving current regulations is whether any of the replacement chemicals will be more harmful to the environment as a whole than CH₃Br, and if so, what steps can be taken to continue its use but eliminate the negative environmental impacts. If the issue is mainly to keep CH₃Br out of the atmosphere, then approaches other than banning use of the chemical may be equally suitable. In fact, providing incentives to develop technology that minimizes negative characteristics before they become environmental or public health problems is certainly a desirable overall approach to any environmental regulation. As scientists we must continue to find methods to protect the environment. In the case of CH₃Br, considering alternatives to a ban on its production and use might minimize negative effects to the economy or food supply as well.

References

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