State of California
M e m o r a n d u m
|To:||Paul H. Gosselin, Assistant Director||Date: 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.
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
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