Submitted to: Journal of Environmental Science and Technology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: November 15, 1996
Publication Date: N/A
Interpretive Summary: Methyl bromide, a soil fumigant, is under intense scrutiny due to evidence which suggests that it damages the stratospheric ozone layer. Because of this, methyl bromide is scheduled for phase-out by 2001. The National Agricultural Pesticide Impact Assessment Program has determined that there will be substantial adverse economic impacts on the agricultural community if the use of methyl bromide is restricted. This has prompted numerous scientists to: study the environmental fate and transport of methyl bromide; search for replacement chemicals and/or nonchemical alternatives; and develop new methodology which improves containment of methyl bromide (or any alternative fumigant) to the treatment zone, while maintaining adequate pest control. This paper provides a comparison between two recent experiments to measure methyl bromide emissions from agricultural fields. The first experiment is a typical shallow application with plastic covering the soil surface, and the second experiment a deep injection without a plastic cover. To date, there have been no comprehensive studies, conducted under typical field conditions, to obtain methyl bromide emissions rates after injecting deep into soils. The results from this study, therefore, offer important new information which can be used to improve pesticide management and reduce atmospheric emissions.
Technical Abstract: An experiment to investigate the environmental fate and transport of methyl bromide is an agricultural field after injection at 68 cm is described and compared to a recent field experiment where methyl bromide was injected at shallow (i.e., 25 cm) depth and the surface covered with high density polyethylene plastic. The methyl bromide volatilization rate was determined for 22 days after application at a rate of 322 kg/ha (i.e., 1134 kg of active ingredient) in a bare field. Only 4% of the total methyl bromide lost from the field occurred after the 10th day of the experiment. Three independent methods, i.e., the appearance of soil Br-, the flux chamber, and micrometeorological methods were used to estimate the total methyl bromide lost after application. The total emissions using the Br-data was estimated to be 239 kg or 21% (+3%). The total emission calculated using the direct flux (i.e., chamber and micrometeorological) methods were found to be in the range of 1.9% to 4.9% of the applied mass. The average mass recovery using all the direct flux methods was 933 kg which was 82% of the applied mass (i.e., 1134 kg). The range in the mass balance percent (i.e., percent of applied mass that is measured) is from 81% to 84%. The discrepancy between Br- and direct flux measurements was attributed to losses from the shank fracture during or immediately after injection. It is important to understand how deep injection affects methyl bromide emissions so that the total fumigant loss from an agricultural field can be minimized.