State of California

M e m o r a n d u m

To:Paul H. Gosselin, Assistant DirectorDate: July 18, 1997
Division of Enforcement, Environmental
Monitoring, and Data Management
From:Department of Pesticide Regulation- 1020 N Street, Room 161
Sacramento, California 95814-5624




The Department of Pesticide Regulation (DPR) conducted this study to determine the effectiveness of buffer zones for methyl bromide applications under winter climatic conditions. Higher air concentrations may occur during winter months due to more stable atmospheric conditions. Hence, methyl bromide buffer zones may need to be adjusted during winter months. To determine the effectiveness of the buffer zones, air concentrations were monitored for four different application methods between December 1996 and February 1997. Each method was monitored one time while a previously unmonitored method was monitored on three occasions.

DPR and the county agricultural commissioners implemented permit conditions, including buffer zones, to mitigate unacceptable methyl bromide exposure. DPR relied upon monitoring data and computer modeling to determine the buffer zone distances. Monitoring to validate the size of the buffer zones was done in the summer months. DPR felt that summer months represented worst-case conditions since more methyl bromide would be emitted in warmer weather because the emissions are governed by adsorption and diffusion through soil. However, concerns about the effect of wintertime weather stability prompted DPR to evaluate these applications.

Study Methods

Between December 1996 and February 1997 air concentrations of methyl bromide were monitored for four different application methods: 1) hot-gas, 2) tarpaulin-bed, 3) very-high-barrier tarpaulin, and 4) shallow tarpaulin. Air samplers were placed at eight sites surrounding each application--one sampler at each corner, and one at the center of each side of the application. The samplers at the perimeter of the application were placed at the buffer zone distance specified by the permit conditions. Air sampling was initiated at the start of the application and continued for 24 - 48 hours.


Air concentrations of methyl bromide exceeded target exposure value [0.21 parts per million (ppm) 24-hour time-weighted average] at the perimeter of the buffer zone in four out of six applications. The highest concentrations were detected for the hot-gas application; two of the three hot-gas applications exceeded the target concentration at the buffer zone. Staff observed problems with the first application such as leaking manifolds. The application may have been conducted inconsistent with appropriate procedures. The other two applications which exceeded target concentrations involved the tarp-bed and very-high-barrier tarp applications. Air concentrations of methyl bromide were within the target exposure value from the shallow tarpaulin application. Weather data taken at the time of the applications confirmed that monitoring occurred during periods of stable weather.


The study demonstrated that weather stability during winter months increases methyl bromide concentrations. The study revealed higher concentrations for hot-gas, tarp-bed and very-high-barrier tarp applications than expected. The results for the tarp-bed and very-high-barrier tarp conflicted with results during summer conditions.

The study highlighted that DPR needs to continue to evaluate management practices implemented to reduce the public's exposure to methyl bromide. DPR will evaluate and implement mitigation measures based on the results of this study.

A complete description of the monitoring is attached.

John S. Sanders, Ph.D., Chief
Environmental Monitoring and Pest Management Branch
(916) 324-4100