Methyl Bromide Volatility Measurements from Treated Fields.

Michael S. Majewski, James E. Woodrow, and James N. Seiber. U.S. Geological Survey, Placer Hall, 6000 J Street, Sacramento, CA 95819.

Methyl bromide is used as an agricultural soil fumigant and concern is growing over the role it may play in the depletion of stratospheric ozone. Methyl bromide is applied using various techniques and little is known about how much of the applied fumigant volatilizes into the atmosphere after application. The post- application volatilization losses of methyl bromide from two fields using different application pracuces were measured using an aerodynamic-gradient technique. One field was covered with a high-barrier plastic film tarp during application and the other was left uncovered, but the furrows made by the injection shanks were bedded over. The cumulative volatilization losses from the tarped field were 22% of the nominal application within the first 5 days of the experiment and about 32% of the nominal application within 9 days including the one day after the trap was removed on day 8. The nontarped field lost 89% of the nominal application by volatilization in 5 days. The error associated with each flux measurement, as well as variations in daily flux losses with differing sampling period lengths show the degree of variability inherent in this type of study.