Methyl Bromide in Castroville, CA

Preliminary Methyl Bromide Monitoring Results

Revilla Drive, Castroville, Calif.


Air monitoring by Environmental Working Group after methyl bromide fumigation of a strawberry field near Castroville, Calif., detected peak levels of methyl bromide exceeding the state Department of Pesticide Regulation health standard in the back yards of residents of nearby Revilla Drive.

EWG's measurements, conducted with instruments believed to be more sensitive than DPR's, were consistently higher than the ones released by the agency. They constitute further evidence that DPR's regulations intended to protect residents from methyl bromide exposure are not only inadequate but flawed in principle, as methyl bromide drift appears extremely difficult to control, even when the pesticide is applied under the tightest restrictions.

Round 1: July 28-30, 1997

The first round of fumigation took place on July 28th, 1997. 10 acres were fumigated with 300 pounds of methyl bromide/chloripicrin mixture per acre for a total application of 3,000 pounds of fumigant applied.

Findings by DPR

Three days later, DPR released a summary of their results in a memo from Paul Gosselin of DPR's Enforcement and Monitoring Division to Department Director James Wells. The memo, "Results of Methyl Bromide Monitoring in Monterey County," stated that all methyl bromide detections were "consistent with what previous methyl bromide monitoring studies have found, and what would be predicted at these distances."

DPR set up a ring of monitoring stations around the fumigated field at distances of 30 and 200 feet. DPR found no methyl bromide at 200 feet upwind of the field, which was predictable, but found methyl bromide at five stations, north, south and east of the field with 24 hour average levels of 24, 141, 153, 189 and 230 parts per billion. All these stations were supposed to be 30 feet from the edge of the field. The 230 ppb finding, although above the state's 210 ppb safety standard, was discounted by DPR because the station was set up 25 feet from the field by mistake instead of at the normal 30 foot buffer zone mark. DPR did not monitor at 200 feet downwind (on the east side of the field).

DPR's brief analysis of these findings was that the levels measured were close to those predicted by computer models and therefore, according to DPR, at safe levels of exposure. Based on this finding, the DPR stated that the 2nd round of fumigation should be allowed to proceed.

EWG analysis of DPR findings

A significant omission from the DPR memo was the fact that the data reported was actually just the first 24 hours of results. They continued monitoring for another 24 hours after this period. Nowhere in the memo is this fact mentioned. Nor is it stated that the reported 24 hour averages might change when the full data are available. The DPR 24-hour methyl bromide standard applies to the peak concentration from any 24-hour period. In previous studies where DPR has conducted 48 hours of monitoring, they have reported the peak average of all samples taken, not exclusively the first 24 hours.

DPR conducted 48 hours of monitoring from 6 a.m. July 28 to 6 a.m. July 30. The results they reported on July 31 were an average of three samples taken from 6 a.m. July 28 to 6 a.m. July 29: a 6-hour sample from 6 a.m. to noon, a 6-hour sample from noon to 6 p.m. and a 12-hour sample overnight from 6 p.m. to 6 a.m. The remaining 24 hours of data and individual datapoints are still not available from DPR.

EWG findings

EWG began monitoring the air near the Revilla Drive field at 6 p.m. on the day of the fumigation (July 28) and monitored in two consecutive 12-hour periods spanning the next 24 hours. Past studies have shown this to be a peak period of off-gassing from fumigated fields. Our Summa canisters were set up side-by-side with DPR 12 hour charcoal samples at four of their stations. We also monitored with the same charcoal filter method that DPR used at two stations.

Stainless steel Summa canisters are thought to be more reliable than charcoal filters for measuring methyl bromide concentrations in air. This part of our study was intended to compare charcoal and canister monitoring technology.

At three of our stations, upwind of the field (to the west), we detected no methyl bromide -- consistent with DPR results. However, at our one station downwind of the field -- the station DPR set up 25 feet from the field -- we detected methyl bromide at 390 ppb and 350 ppb in consecutive samples, for an average of 370 ppb over 24 hours. This finding is 60 percent higher than the 230 ppb average DPR reported for this same station, and well above the 210 ppb safety standard.

DPR has yet to release the individual data points from their sampling and has only reported averages, so there is no way yet to compare our Summa to their charcoal samples. Our charcoal samples are still being analyzed by our lab contractor.

Round 2: Aug. 1-2, 1997

On the basis of their results from monitoring the first application, DPR allowed the second round of fumigation to proceed on Aug. 1. DPR conducted charcoal-tube monitoring of this fumigation as well, at 17 sites around the field. Seven samplers were 30 feet from the edge of the field; one sampler was 440 feet east of the field; two samplers were 60 feet west of the field, two were 100 feet west, and five were 200 feet west, at the property line of the Revilla Drive residents.

According to an Aug. 15, 1997 memorandum from Gosselin to DPR Director James Wells, "None of the results at the 200-foot buffer zone distance exceeded the target volume of 210 ppb. The highest 24-hour time-weighted average detected at the 200-foot buffer zone was 199 ppb. However, at four locations inside the [200-foot] buffer zone, the 210 ppb level was exceeded."

EWG analysis of DPR findings

Again, the summary released by DPR makes a detailed analysis of the agency's second round of monitoring difficult. Although DPR monitored for 48 hours following the application, in segments of two six-hour periods and three 12-hour periods, the summary discloses only the highest 24-hour averages for the 17 samplers. The summary does not specify which 24-hour period was used for the averages, nor what levels were detected during the shorter time segments. Gosselin's memo says DPR "will complete a detailed report on these findings in the next few weeks."

EWG findings

The purpose of this phase of our study was to monitor more intensively the drift of methyl bromide outside the DPR buffer zone into people's backyards. EWG set up monitors (both Summa canisters and charcoal filters) at four stations on the west side of the fumigated field, including two in the back yards of Revilla Drive residents. The fumigation was supposed to be forestalled if wind was blowing toward the homes on Revilla Drive and was delayed two hours for this reason, but then finished the same day.

All samples were taken between the field and the houses on Revilla Drive. The station locations were:

Station 1: 30 feet from the edge of the field at the center along the west side.
Station 2: 30 feet from the edge of the field at the southwest corner.
Station 3: In the backyard of 15053 Revilla Dr., 236 feet due west of field.
Station 4: In the backyard of 15047 Revilla Dr, 248 feet from the southwest corner of the field.

Results (parts per billion):

  8/1-2/97 8/2/97  
Station 6 pm-6 am 6 am-6 pm 24 hour avg.
1 480 220 350
2 180 no sample 90
3 490 no sample 245
4 290 28 159


In the backyard of 15053 Revilla Drive (Station 3), EWG's measurement exceeded the 210 ppb 24-hour standard. Our Summa canister sample was 12 hours in length and measured 490 ppb. Using DPR's formula, even if no methyl bromide at all was in the air during the other 12 hours, the 24-hour average at this site would be 245 ppb.

At Station 4, in the backyard of 15047 Revilla Drive, we also got a high reading of 290 ppb for the first 12 hours. The 24-hour average for this station was 159 ppb.

At station 2, also 30 feet from the field, during the first 12 hours we detected an average of 180 ppb. No EWG sample was taken at this station during the second 12 hours, making the estimated 24-hour average 90 ppb.

At Station 1, at 30 feet, the normal buffer zone distance for this type of fumigation, we detected an average of 480 ppb in the first 12 hours and 350 ppb for the full 24 hours -- 67 percent higher than the state standard.

Interestingly, we detected levels at Station 3 that were higher than Station 1, which was only 30 feet from the field in the same orientation -- something that could be accounted for by the swirling winds present at the site that day.

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