tt 8. ne Fee 4 (il through 20). Underground tests at the NZS must always be viewed as a potential source of radioiodine for milk within the U. 5. because of its proximity compared with any proving ground cutside the continental U. S. rurther, the radioactivity from uncerground tests at NTS may often be released near the ground so that direct contact with the crovs and scavenging by precipitating clouds is easier than for nuclear clouds aloft. Finally, since qrst is gaseous, atmospheric contamination from underground tests may be postulated even when particulates fail to escape. Questions about the source of radioicdine have been raised for the period in 1961 and 1962 when both etnsospheric and underground tests were being conducted. Over five and one helf years have elapsed since the I zsh fallout from the 1961-62 atmospheric nuclear test stopped and there are now intervals with frequent underground but no atmospheric nuclear tests. inspection of Fig. 1 reveals nine such int-ervals which are listed in Table 2b, during which 5 cratering and 133 contained underground testswwere reoorted to have taken place within the United States. underground tests were reported to have vented. In addition, tL The term "vented" is used to mean that radioactivity was detected off the Nevada Test Site. Excluding the interval after the Palanquin cratering event of 14 April 1965 and the unexplained November-December 1965 episcde in southeastern United States (to be discussed later), no ist concentration in milk of the PMN exceeded 30 pci/2l. Because of the cattle feeding practices during cold weather, a better measure of the lack of radioiodine in milk following underground tests in the absence of atmospheric nuclear tests or the Palanquin event may be obtained from the warm seasons, May through October. Examination of these warm season PMY milk concentrations also shows that no milk sample contained over 30 pci/1 despite 64 reported underground tests of which three were reported to have vented (Table 2c). During the periods of rt) eariout from atmospheric tests in 1963-68 there were also 30 reported underground tests in the United States, four - of which were reported to have vented. There remains the possibility that these underground tests contributed ti3L to she PMN milx which may have been masked by atmospheric testing. ~% is unlikely, however, that under- ground tests will produce zi3l in PMN milk only when there are atmospheric tests. a_i y—