The California Environmental Protection Agency's Department of Pesticide Regulation (DPR) is well into its 6-month effort to monitor methyl bromide applications throughout the state. Scientists working on the project, which began in July 1997 and goes through February 1998, monitor about two fumigations each month. They set up buffer zones between fumigation sites and residential areas to measure levels of methyl bromide in the air 48 hours after application. Meteorological data are being collected to measure wind speed, wind direction, ambient temperature, and relative humidity.

This work is a result of action taken by DPR and California county agricultural commissioners in 1993 to ensure better protection for workers and others who may be exposed if methyl bromide escapes fumigation sites. A buffer zone is the area between the edge of a field treated with methyl bromide and nearby occupied buildings or land areas, like parks. The zone extends in all directions around the treated area. Buffer zones are set so that concentrations measured at this distance don't exceed an average of 210 parts per billion (ppb) over 24 hours—0.21 parts per million (ppm)—which builds in a 100-fold margin of safety for possible exposure. This level is 100 times lower than safe exposure levels established by toxicology tests.

California is the only state thus far to adopt these safety measures, which far exceed national standards. The safety measures are based on more than 1,000 field-measured air samples from 11 fumigations where the buffer zone provided at least a 100-fold safety margin. Although these tests were done in the summer, DPR did additional monitoring during the winter of 1997 to evaluate methyl bromide air concentrations in cold, stable air conditions.

"Because of cold air possibly draining down a slope and causing higher methyl bromide concentrations, we required a buffer zone of 200 feet for the side of the field that slopes toward residences," explains Paul H. Gosselin. "And, we established a 30-foot buffer on all other sides."

He is assistant director of DPR's Division of Enforcement, Environmental Monitoring, and Data Management.

In Monterey County, DPR monitored two separate applications to a 22-acre field where methyl bromide was injected 12 inches beneath the soil surface by shanks attached to a tractor. Growers had applied 300 pounds per acre of a mixture of 80 percent methyl bromide and 20 percent chloropicrin, then covered the treated area with a very high barrier tarpaulin.

Air samplers with activated charcoal tubes were placed around the treated area and wind speed and direction and relative humidity were recorded every 5 minutes. Air samples were taken from 13 locations around the field in the first application and 17 in the second. A series of 5 samples was collected from each of the locations beginning with the start of fumigation during two 6-hour periods and three 12-hour periods for a total of 48 hours. Samples were analyzed by the California Department of Food and Agriculture's Center for Analytical Chemistry.

"During the first application, methyl bromide was not detected 200 feet from the edge of the field. Our sampling method can detect concentrations as low as 10 ppb," Gosselin reports. "During the second application, however, levels as high as 0.199 ppm were detected 200 feet from the edge of the field."

Although the methyl bromide detected didn't exceed the target level of 210 ppb at the buffer zone distances for either application, levels were higher than expected. Based on preliminary data, Gosselin thinks this could be attributed to weather factors. Wind speed during the night following the first application was less than 3 miles per hour for 3 of the 12 hours monitored. The night following the second application, wind speed was less than 3 mph for 5 of the 12 hours. Usually, the lower the wind speed, the higher the air concentration. Also, there was an overcast sky throughout the night of the first application, but clear skies during the night of the second application. Gosselin says that clear skies create more stable atmospheric conditions with less mixing of the air.

"All other factors being equal, a more stable atmosphere produces higher methyl bromide concentrations in the air," he says.

DPR has preliminary results from the first application at monitoring sites in Ventura County. In August, about nine acres were treated with 225 pounds per acre of an 80-20 mix of methyl bromide-chloropicrin. Weather data were again collected from a temporary weather station on site, and air samples were taken at 17 sites around the field. Eight samples were taken 30 feet from the field, 4 from 60 feet, 3 from 100 feet, and 2 from 200 feet.

"None of the samples showed levels of methyl bromide in excess of the 0.21 ppm target," Gosselin reports. As a result of the monitoring, DPR allowed the applications to this field to proceed but set a 100-foot buffer zone between it and an adjacent trailer park.

"DPR will change permit conditions for applying methyl bromide in the winter. At a minimum, we'll lengthen the buffer zones," Gosselin says.