Controlling Methyl Bromide Emissions with Impermeable Films

Methyl bromide is critical to agriculture worldwide as a soil fumigant, a postharvest storage treatment, and a quarantine treatment to control many pests on various crops. The primary use is to fumigate the soil to destroy soilborne pests, but unfortunately, some of the chemical escapes from the soil into the atmosphere. Methyl bromide has been declared an ozone depletor and, under provisions of the U.S. Clean Air Act, will be banned from production and importation in the United States on January 1, 2001.

But, scientists with USDA's Agricultural Research Service and the University of California have developed new technology that can greatly reduce methyl bromide emissions from soil fumigation.

"In a recently completed field experiment, we used a virtually impermeable film (VIF), which we left on the field for 10 to 15 days, and reduced methyl bromide emissions up to 96 percent," reports Scott R. Yates. Yates and ARS colleagues Dong Wang, Jay Gan, and Fred Ernst, with the U.S. Salinity Laboratory in Riverside, California, worked with William A. Jury, University of California-Riverside, on the project.

"The new plastic seems to have physical and mechanical properties similar to conventional plastic, so it can be used readily in the field," Yates says. "Growers would need to make very few changes in their fumigation practices."

In a recently released report, the Environmental Protection Agency estimates that the new films would cost about 47 percent more than conventional films. That, Yates says, is based on the fact that the films are not produced in the United States and are not yet produced in mass quantity. "Mass production would probably bring the cost of the VIF films more in line with what growers are now paying for conventional films."

Growers now pay between $275 and $314 per acre for conventional, or high-density, polyethylene film (HDPE). VIF films are estimated to initially cost about $580 per acre.

According to Yates, tests in the laboratory showed that the new plastic film—Hytibar™—is at least 75 times more effective at keeping methyl bromide gas in the soil than conventional plastic films. "When we repeated these tests in fields, Hytibar™ was about 200 times more effective than the plastic now being used."

Yates says the wide range is due to difficulty in measuring the extremely low permeability of the new film. "We're working on ways to more accurately measure film permeability."

"Soil is conventionally fumigated under HDPE film. Using the new VIF film for 10 to 15 days could cut the global methyl bromide emissions from soil fumigation from 33 percent to less than 1 percent of the total global sources," Yates says.

Emission Rates: Old Films

Growers typically inject methyl bromide in the soil at the rate of 250 lb/acre, about a foot deep, and cover the field with HDPE plastic for 2 to 5 days. Yates and colleagues conducted an experiment testing three scenarios: HDPE film with an application rate of 250 lb/acre (standard rate of application), VIF at 188 lb/acre (75 percent of the standard rate) and VIF at 125 lb/acre (50 percent of standard).

With HDPE, total emission losses were between 56 and 68 percent of the original amount applied. "These large losses are primarily due to the ineffectiveness of the plastic film in trapping gases," Yates explains. "Growers use this type of film because it's relatively inexpensive and easy to use."

The variability in the rate of loss can be due to differences in soil and environmental conditions indigenous to the location of the experiment and in the ways of measuring the emissions rate. Ambient temperature during fumigation and local variations in soil degradation are principal factors for the wide range in reported total loss. According to Yates, permeability of HDPE films is strongly temperature dependent and increases from 1.5 to 2 times for each 18 °F temperature increase. How well the soil breaks down methyl bromide depends on the soil type and organic matter content.

Emission Rates: New Films

Total emission losses from fields covered with VIF for 5 days were reduced to 36–39 percent with most of the loss occurring immediately after the plastic was removed from the soil surface. "When we left the new plastic on the fields for 10 to 15 days, total methyl bromide losses were less than 4 percent," Yates reports. The accuracy of the 4 percent emission rate was confirmed by measuring soil degradation of methyl bromide 43 days after application; the soil degraded 96 percent of the methyl bromide applied (±2 percent).

The new plastic, manufactured by Klerk's Plastic in Belgium, is made by putting a barrier polymer (ethylene vinyl alcohol) between two layers of polyethylene. This makes the new film less permeable and therefore better able to keep the chemical from escaping into the air. Ultimate emission rates are greatly affected by the soil's ability to degrade methyl bromide. Once methyl bromide breaks down, it releases a bromide ion (Br) into the soil and is no longer harmful to the atmosphere. Yates says that traditionally used plastics are too permeable to keep the chemical in the soil long enough for it to fully degrade.

"We noted that for the plots covered with VIF, the same fractional percent of total volatilization occurs regardless of the initial application rate," Yates says. "From this we can estimate how much methyl bromide would be lost if VIFs were used instead of HDPE."

Yates and colleagues also showed that methyl bromide application rates could be reduced if VIFs were used. Since very little of the chemical escapes, the standard application can be decreased while maintaining the same level of pest control. And, using reduced application rates would further reduce methyl bromide emissions into the atmosphere from soil fumigation.

According to Yates, there are serious questions about whether the methyl bromide ban will have any significant effect on stratospheric ozone levels. It has been suggested that the oceans act as a buffer by contributing methyl bromide to the atmosphere which will offset any reduction. And, a recently completed ARS study shows that plants living in soils containing the bromide ion may produce significant quantities of methyl bromide. This could mean a loss to the agricultural community and society, since the ban will wipe out an effective soil sterilant but do little to stop ozone depletion.

"We plan further research on VIF films at different locations and under different soil and environmental conditions," Yates says.

Last Updated: April 22, 1998