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United States Department of Agriculture

Agricultural Research Service

Emission Reduction
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1 - Introduction
2 - Containment
3 - Soil Degradation
Containment is necessary to hold the gas at the treatment location and provide sufficient fumigant concentrations and time for pest control. Without adequate containment, a large fumigant quantities will be lost to the atmosphere. Containment is difficult because of high vapor pressures and low boiling points, which result in a large fractions of a fumigant existing in the vapor phase at normal temperatures and pressures. Movement in the vapor phase occurs more rapidly compared to the liquid phase due, in part, to a larger gas-phase diffusion coefficient compared to the liquid phase. Pesticides which have a large vapor pressure easily move through soil (Goring, 1962; Kolbezen et al. 1974; Reible, 1994). Factors that affect containment include: use of plastic, the properties of plastic, injection depth, soil bulk density, soil water content, soil cracking, and other mechanisms which promote or retard movement.
Plastic-Film Barriers
Plastic films are commonly used to improve containment and reduce the amount of fumigant leaving the treated soil. Covering soil with plastic film, or tarping, was introduced early in the development of soil fumigation with MeBr to enhance efficiency and reduce toxicity to workers and residents. Covering the field with plastic can reduce volatilization by inhibiting transport from the soil into the atmosphere. In early studies, it was shown that tarping increases fumigant retention in soil. Kolbezen et al. (1974) reported increased MeBr concentrations in surface soil under tarped conditions, but they later found that, overall, polyethylene film was a poor cover material compared to other plastics (Kolbezen and Abu-El-Haj, 1977). The ineffectiveness of polyethylene tarps in preventing MeBr volatilization was realized later by many other workers (de Heer et al., 1983; Rolston and Glauz, 1982; Jin and Jury, 1995). This ineffectiveness is not only caused by the high permeability of the polyethylene film to MeBr, but also by the short covering time normally used in agricultural fumigations.
Advantages of using films are that the properties and condition of the film are known in advance and films are more uniform in space and time compared to soil. There may be a higher certainty of effective containment when films are used compared to soil-water based methods. Also, the level of containment can be controlled by altering the plastic material used. New plastics are available which are very impermeable to fumigant diffusion (i.e., resists a fumigant passing through the film). This has led to the need for methods to estimate film permeability. The effect of various plastic materials and ambient temperatures on film permeability have been investigated. Low-permeability films have been developed which have permeability hundreds to thousands of times lower than that of 1-mil HDPE. For more information see Film Permeability.
Studies have shown that increasing the soil water content under soil tarps decreases cumulative emissions of soil fumigant compounds. Slower transport from the application depth to the soil surface in soils with high moisture increases soil degradation. In a soil column study where the soil surface was tarped with PE and the column was exposed to diurnal temperature variations, soil water accumulated at the soil surface; this accumulation resulted from condensed water on the tarp being redeposited on the soil surface and from diurnal heat variation resulting in upward flux of water vapor at night (Jury et al., 1996). Increased water content at the soil surface had a large impact on cumulative emissions in PE-tarped columns, so that the application of water under 1-mil HDPE was much more effective at reducing emissions than was surface sealing with 1-mil HDPE alone (Jin and Jury, 1995).
Using a simulation model, Reible (1994) showed that volatilization losses would be reduced from 53 to 33%, following a 25 cm injection, if the cover time was extended from 2 to 7 days. The total volatilization loss would depend heavily on the selected degradation rate. In packed columns, covering the soil surface with 0.025 mm high-density polyethylene film for 2-3 weeks generally reduced volatilization loss by 40% (Gan et al., 1997a). Less permeable plastics have also been tested (Kolbezen and Abu-El-Haj, 1977; de Heer et al., 1983; Gamliel et al., 1997; Chakrabarti et al., 1995; Daponte, 1995; Wang et al., 1997a; Wang et al., 1998; Papiernik et al., 2001), and their usefulness in reducing volatilization has attracted attention over the last few years.
Wang et al. (1997a) demonstrated a large reduction in MeBr volatilization using reduced application rates and high-barrier plastic films. In Hytibar-tarped plots, less than 4% of the applied MeBr volatilized when the tarp remained on the soil surface for at least 10 days. In polyethylene-tarped plots, MeBr emissions were less than 50%(Wang et al., 1997a).
Injection Depth Total Emissions (Measured)(%) Total Degradation(%) Mass Balance(%) Total Emissions Corrected Using Diffusion Model† (%)
Tarped Soil Columns
20 cm 59 36 94 43
30 cm 52 39 91 37
60 cm 45 46 91 26
Non-Tarped Soil Columns
20 cm 91 12 102 82
30 cm 83 15 98 71
60 cm 60 36 96 38

This Table provides a summary of the total MeBr emission in percent of applied MeBr for both tarped and untarped treatments following injection into soil columns (Gan et al., 1997a). These results indicate the importance of injection depth and use of a plastic tarp in reducing MeBr volatilization.
When the soil surface remained bare, MeBr volatilization was extremely rapid (Majewski et al., 1995), with as much as 80-90% of the total loss occurring during the first 24 h. When a tarp was present on the soil surface, the maximum volatilization flux was significantly smaller, with only 30 - 45% of the overall loss occurring during the first 24 h.
Surface Barrier Laboratory Columns Field Plot Experiments Field Experiments
Cover Period (days) Total Emission (%) Cover Period (days) Total Emission (%) Cover Period (days) Total Emission (%)
Bare na 44-90 na 87 na 89
HDPE 8 37-83 15 67 4,5,8 32-67
Hytibar 8 2 15 <5 na na

This Table shows recent estimates of total MeBr emissions for soils left bare and covered with plastic film after application. It is clear that the use of HDPE has a beneficial effect on reducing MeBr emissions, and using virtually impermeable films (Hytibar) appears to hold great promise for reducing emissions to near-zero levels.
Application Depth
The depth of application is an important factor affecting the amount of fumigant escaping into the atmosphere. In laboratory soil columns, when the application depth was increased from 20 to 60 cm, the MeBr emission rates decreased by 54% under untarped conditions, and 40% under tarped conditions (Table 1 above). Combining deep injection with the use of a surface tarp has the potential to significantly reduce MeBr emissions (Table 1 above). This supports the results from a recent field experiment (Yates et al., 1997), where 21% of the applied mass was volatilized when MeBr was injected deep in the soil (68 cm) and the soil surface left uncovered. This is 66% less than the total MeBr emissions rate (62%) observed in an adjacent field for which shallow MeBr injection and a high-density polyethylene tarp was used (Yates et al., 1996a,b,c). The lower emissions rate may be attributable to the deeper injection depth and a cooler average air temperature.
These results are also in agreement with the predictions made by Reible (1994) using a vapor transport model. Under hypothetical conditions, it was estimated that increasing the injection depth from 15 to 45 cm would decrease the MeBr emission rates from 53% to 28% when the soil was tarped. From these findings, it can be concluded that placing MeBr at a greater depth is another effective approach for minimizing its emission into the air during soil fumigation.
Soil Water Content
Increasing soil water content has been considered as a means for controlling fumnigant movement (Goring , 1962; Reible, 1994; Jin and Jury, 1995). The effect of water content on volatilization can be explained by the interactions of soil water content and the retardation factor, R d= (q + K dr b)/K h+ e, and tortuosity factor, (e.g., t = e 10/3/h 2) in gas-phase transport, where h, q, r b, K d, K h, respectively, are the porosity, water content, bulk density, liquid-solid and liquid-gas partition coefficients and the air content is e = h-q.
When the water content in laboratory columns containing Greenfield sandy loam was increased from 0.058 to 0.180 cm 3cm -3(i.e., v/v), R dincreased from 1.21 to 1.58, t decreased from 0.241 to 0.076 and the effective soil diffusion coefficient was reduced by 76%. For volumetric water contents of 0.058 and 0.124 (v/v), the estimated emission loss after correcting for the presence of the column bottom was approximately 77% of the applied MeBr (Gan et al., 1996; Table 9). This indicates that at moderately dry soil conditions, water content does not strongly effect emissions. When the water content was increased to 0.180 (v/v), only 62% of the applied MeBr was lost. As the soil water content increased, the maximum MeBr flux density decreased and the time interval before reaching the maximum flux density increased. Measurements of the MeBr gas concentration in the soil also indicated rapid movement through the soil column for the drier soils. MeBr in these soil columns was completely depleted 54-72 h after the application. For the wetter soil, measurable concentrations remained in the column until 144 h after the application.
In a field experiment (Yates et al., 1997), lower MeBr emissions were observed for bare soil, deep application than for a tarped, shallow application in the same field. Part of this difference may have been attributed to the water content of the soil profile. During the deep-injection study, the average soil water content around the injection point (68 cm below the surface) was 0.223 (v/v), whereas that observed during the shallow-injection study was 0.145 (v/v). Further, light rain occurred during the early part of the experiment which helped to seal the soil pores. Although the measured water content data were not given, Yagi et al. (1995) also attributed the decrease in MeBr emission from 87 in their first study to 34 % in their second study, in part, to soil moisture differences. Similar results were observed in the laboratory by Jin and Jury (1995).
Soil Bulk Density
Soil bulk density can also have an effect on MeBr transport since the pore space decreases as bulk density increases. The bulk density, r b, is related to the porosity, h, from the relationship: h = (1 - r b/r p), where r pis the particle density.
In laboratory columns packed with Greenfield sandy loam (Gan et al., 1996), the corrected cumulative volatilization loss for a column with a bulk density of 1.70 g/cm 3was 53%, significantly lower than the 77% loss from a column with a bulk density of 1.40 g/cm 3. The columns with higher bulk density behaved in a manner similar to the wetter soil column described above. Measurable volatilization continued for 120 h, the maximum flux density was reduced ~60% compared to the low bulk density column, and the time to reach the maximum flux increased to 6.5 h after application (compared to 2.5 h in the low bulk density column).
In the untarped, deep-injection field study (Yates et al., 1997) the field was disked and packed shortly (approximately 5 min) after MeBr was injected into the field. The disking and surface packing closed the openings above the injection fractures and increased the bulk density near the surface. This, along with a higher water content, probably contributed to the reduced total emission compared to the shallow-tarped experiment. Packing the soil surface and carefully closing the soil fractures created during application should be considered for minimizing MeBr volatilization.
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Last Modified: 11/4/2009
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