Submitted to: Journal of Environmental Quality
Publication Type: Peer reviewed journal
Publication Acceptance Date: 7/14/1996
Publication Date: N/A
Citation: Interpretive Summary: Methyl bromide has been used for decades as a soil fumigant for the control of nematodes, weeds and fungi and has recently come under scrutiny as a chemical which depletes stratospheric ozone. As a result of the Clean Air Act, methyl bromide is scheduled for phase-out by the year 2001. Restricting the use of methyl bromide will cause substantial adverse eeconomic impacts on the agricultural community. A research project was initiated to determine the extent of methyl bromide movement from the soil into the atmosphere. The primary objective was to assess the potential impacts on the environment from the agricultural use of methyl bromide. The field experiment was conducted in an 8 ha field after methyl bromide was applied at a shallow depth under a 1-mil polyethylene tarp. This application method is commonly used to treat soil prior to planting vegetables. The methyl bromide mass degraded to Br ion and the mass remaining in the field at the completion of the experiment. This allows a estimate of the total methyl bromide emission from the field to be determined. An important distinction between the experiment described herein and previous experiments involving methyl bromide is obtaining the necessary information to report the mass balance; principally, the mass of methyl bromide converted to Br ion. Previous experiments measuring methyl bromide flux failed to provide this vital information along with the mass of methyl bromide remaining in the field, making it impossible to determine whether the estimated emission rates from agricultural fields were accurate.
Technical Abstract: An experiment is described to investigate the environmental fate and transport of methyl bromide in agricultural systems. The experiment was designed to determine the dynamics of methyl bromide movement through soil, degradation and total emissions to the atmosphere. This is of particular interest because it will allow an assessment of the environmental impacts (i.e., stratospheric ozone depletion) resulting from the agricultural use of methyl bromide. Methyl brome was applied at a rate of 843 kg in a 3.5-ha (i.e., 240 kg/ha) field at a depth of 0.25m and covered with a sheet of 1 mil polyethylene plastic. The maximum methyl bromide concentration in the atmosphere occurred at night between 0200 and 0600 hours. During the first three days of the experiment, the concentrations at 0.2m above the soil surface were 30, 5 and 1 mg/m3, respectively. The trend of reduced emissions with time continued until the plastic wsa removed, when a momentary increase in the methyl bromide emissions occurred. The maximum soil gas concentration after 24 hours was 30 g/m3 located at 0.25m depth. When the plastic was removed from the field (at 5.6 days), the maximum soil gas concentration was approximately 2 g/m3 at 0.5 m depth. A mass-difference method for estimating the total methyl bromide emissions from the soil, based on degradation of methyl bromide to Br ion, indicates that approximately 39% or 330 kg (+/-157 kg) of the applied methyl bromide was converted to Br ion and, therefore, 61% or 513 kg (+/-157 kg) was lost via volatilization.