Environ. Sci. Technol. 1997, 31, 3686-3691
Reducing Methyl Bromide Emission with a High Barrier Plastic Film and Reduced Dosage D . W A N G , * ,† S . R . Y A T E S , † F . F . E R N S T , † J. GAN,† AND W. A. JURY‡ U.S. Salinity Laboratory, Soil Physics & Pesticides Research Unit, 450 West Big Springs Road, Riverside, California 92507-4617, and Department of Soil and Environmental Sciences, University of California, Riverside, California 97521
A field experiment was conducted to study the effectiveness of a high-barrier plastic film (Hytibar) and reduced methyl bromide (MeBr) application dosages in decreasing MeBr emission from soil fumigation. Comparison was made with a conventional polyethylene film (PE) and a regular application rate. MeBr emission from nine small field plots was continuously measured with two replicated active chambers. Replicated soil cores were taken to a depth of 3 m to determine MeBr degradation and to obtain a mass balance. Results indicated that about 64% of applied MeBr was lost to atmospheric emission when covered with the PE tarp for at least 5 days. The emission was reduced to 37.5% when covered with the Hytibar tarp for 5 days. MeBr emission was virtually eliminated when covering with the Hytibar tarp for more than 10 days (only 1.1-3.2% loss). The low emission rate was verified from the degradation measurements which, along with the flux measurements, produced nearly 100% mass balance. Combined use of a highbarrier plastic film such as the Hytibar with reduced application rates can significantly reduce MeBr atmospheric emission while maintaining effective pest control efficacy.
Introduction Recent research on methyl bromide (MeBr) emission indicates that 21-87% of applied MeBr is lost to the atmosphere after soil fumigation (1-5), which contributes significantly to stratospheric ozone depletion (6). The large emission loss is caused primarily by the ineffectiveness of polyethylene (PE) tarps used for retarding MeBr emission. Yates et al. (4) found that 36% of the applied MeBr was lost only 24 h after application and over 60% was lost by the time of tarp removal (5 d). The high variability in measured emission loss may be attributed to differences in experimental methods used to measure the emission rate; differences in environmental conditions such as solar radiation, air and soil temperature, ambient barometric pressure, wind speed, and relative humidity that are indigenous to the location where an experiment is conducted; and variability in tarp properties when flux measurement is made at a small scale (2). Because of its complexity and high cost, field measurements of MeBr volatilization rate are rarely replicated. This increases experimental uncertainty as compared to experiments that replicate treatments and/or provide independent volatilization measurements. * Corresponding author telephone: (909) 369-4857; fax: (909) 3424964; e-mail address:
[email protected]. † U.S. Salinity Laboratory. ‡ University of California.
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ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 31, NO. 12, 1997
One approach to reducing MeBr emission uses a film that is less permeable to MeBr than PE. This is an efficient approach because it requires the fewest changes in current fumigation application practices. In fact, it has been found, based on preliminary laboratory measurements (7, 8), that materials such as Mylar, Saran, or aluminum foil are much less permeable to MeBr than the PE films. The PE films are still widely used in soil fumigation because of their suitable physical and mechanical properties for field use and costeffective economic feasibility. A new plastic film (Hytibar, Klerk’s Plastic, Belgium), manufactured by incorporating a barrier polymer (EVOH) between two layers of PE, is claimed to have a very low permeability to MeBr (9). Preliminary tests in the laboratory have shown that this new plastic is about 70 times less permeable to MeBr than the conventional PE films (8). It also appears to have the physical and mechanical properties that are similar to the PE films, making it feasible for field use. Using a film with similar composition to Hytibar, effective control of fungal pathogens was achieved with only 40% of the regular dosage in Israel (500 kg/ha), whereas only 20% mortality was obtained with the PE film and the reduced rate (10). A field experiment with indirect emission estimates also indicated the potential effectiveness of the Hytibar plastic for reducing MeBr emissions (11). The cost for the Hytibar film will probably be slightly higher than the popular PE films because of the relatively expensive additive material (EVOH). Current MeBr application dosage in soil fumigation is established based on many field trials under the PE tarps. In California, MeBr is often applied as a 67/33% or 75/25% mixture with chloropicrin (CCl3NO2), and the total amount of MeBr applied is in the range of 240-320 kg/ha (1-5). Clearly, reducing application rate under the same management methods will automatically lead to a reduction in the amount of atmospheric emission. To maintain pest control efficacy, however, a highly resistant film that blocks atmospheric entry will be required when dosage is reduced to maximize the effectiveness of the fumigant. Although a more protective tarp would increase cost, this might be offset by the reduction in fumigant application costs. The purpose of this study was to experimentally evaluate the effectiveness of a high-barrier plastic film, Hytibar, for reducing MeBr emission with replicated field plots. Three scenarios were tested: (a) standard fumigation practices using a PE film at a 280 kg/ha application rate (100% dosage); (b) Hytibar with a 210 kg/ha dosage (75% of the standard rate); and (c) Hytibar with a 140kg/ha dosage (50% of the standard rate). Each treatment was replicated three times over randomly selected plots, and each plot had replicated direct emission and degradation measurements.
Experimental Section Experimental Design and MeBr Application. Nine experimental plots were constructed in a field located on an University of California Agricultural Experimental Station near the Riverside campus. Each plot was about 3.4 by 4.9 m with four layers of continuous plastic tarp (two layers of Hytibar sandwiched between two PE films) buried vertically to 3 m depth along the perimeter of each plot. This kept MeBr gas from moving laterally away from the treatment zone and simulated large-scale field conditions. A detailed description of plot construction can be found in Wang et al. (11). A Latin square experimental design was used to block any potential variability in soil properties between the treatments (Figure 1). To determine the effect of tarping duration on MeBr emission, tarps were removed from one of the three replicated
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1997 American Chemical Society
FIGURE 1. Latin square experimental design for determining effect of tarp type, dosage, and tarping duration on methyl bromide emission. plots for each treatment at 5, 10, and 15 days after application, which will be referred to as PE5, PE10, PE15 for the three polyethylene plots and HB5, HB10, and HB15 for the Hytibar plots. Emission measurements were continuously made during and after tarp removal and terminated only when no measurable MeBr was detected in the air samples (
