7 milk concentretions of any NTIS event and, as noted earlier, its ist appeared in the PMN milk et Helena, Montana as well. The SWRHL collected ilk samples from 154 locations in the western U. S. for this event with sive sampling concentrated in seven states. The farthest distance st in milk >30 pCi/l was found at Miles Clty, Montana, approximately 900 miles from the test site. The date of this sample preceded that found at Helena in the PMN from the same event. "Siovron or the fifteenunscheavlea releases of radioactivity from underground tests at the NTS produced no detectable zis in the local NTS milk samples. The Pin Stripe event on. April 25, 1966, resulted in a milk concen- tration of 4,800 pci/l at a distance of 60 miles. to 70 pci/l in a sample obtained at 550 miles. Concentrations decreased Aside from Pin Stripe, the > highest concentration of qist in the SWRHL milk network from an unscheduled release of radioactivity from an underground test (130 pCi/1) occurred from the June 16, 1965 event. Actually, at this time, fallout from a Lop Nor nuclear test deposited radioactivity over the United States and the assignment of the origin of the radioiodine in milk is ambiguous (23). ‘The same - eonfusion on the source of radioiog@ine in milk existed a year later for the June 8, 1966 rocket test (24). Why did the PMN not reflect the presence of radioiodine when it was seen in the local SWRHL network? The probable explanations are both meteorological and non-meteorological. In the latter category one notes that the PMN composites milk from farms dispersed over hundreds to thousands of square miles. Milk from ?Parms with detectable concentrations of ist can be diluted by milk from other farms in the milkshed with no radioiodine, * the composite qist being too small to detect. This contrasts with the SWRHL network where, for the most part, individual farrsor dairies are sampled. The clouds from some of the atmospheric releases in Table 3 moved northward in the cold hal? of the year when cows were not on pasture. probaoly more important are the several meteorological reasons. But A cloud of radioiodine dilutes as it moves downwind of its source due to both horizontal and vertical turbulent mixing and to removal processes. The dilution caused by atmospheric diffusion, on the average, decreases the peak concentration in the cloud at the rate of roughly the square of time.” The peak concentration et 5 hours would thorefore be reduced by a factor or 25 one day later. The rate of loss due to upteke of zis by the soil and