\ double peak in strontium-90 levels appears in 1968. Although not apparent from this study other investigators have noted the presence of fresh fission products during August 1968 and attribute the rise in radioagthyity to the rapid movement of debris from the French tests in July 19668. Strontium-90 ievels in the latter part of 1968 would be elevated if such an intrusion occurred. Strontium—-89 concentrations are shown in Table 2. Strontium—89 values are indicated only in months where the levels of this radionuclide were above the minimal detectable level. All the appearances of strontium-89 followed Chinese atmospheric tests. This fact, together with information that, except in the case noted above, French tests were not measurable 10°. latitude (11 and that the reported yields of releases at NTS were low and of a limited nature ° ; appears to make the Chinese detonations the principal source of increases above the expected strontium-90 levels. The contribution of strontium-90 from a particular test may be estimated from the strontium-89 to strontium-90 ratio. These estimates are also presented in Table 2. The theoretical ratios are for uranium-235 fission. (le) In most cases, the contribution of strontium-90 ground level air from a recent nuclear test ET PELee pased on the strontium-89 ratios, appears to be of trospheric origin. However, the strontium-89 to strontium-90 ratio following the fifth Chinese test appears to be influenced by the presence of strontium—29 of stratospheric origin beginning in March of 1967. Early low yield tests contributed only small amounts to ground level strontium-90 concentrations. The fifth Chinese test (December 27, 1966) appears to have contributed a substantial quantity of strontium-90. There was no evidence of a fresh strontium-90 intrusion following the sixth Chinese test (June 17, 1967), although complete data are not available. For the two months following the test, for which results were available, there was no indication of elevated strontium-89 levels, Data from HASL also indicate the cbsence of strontium-89 from this test.™ . From the differences in projected and actual levels of strontium-90, an estimate of the strontium-90 from atmospheric testing after 1963 can be made. | Table 3 presents the mean predicted (based on a 10-month residence half-time and the mean 1964 results) and measured strontium-90 concentrations in ground level air. Because of the magnitude of errors in analyses, it is believed that only the values for 1968 are truly significant, although a gradual increasing trend in newly injected strontium-90 since 1965 is indicated. In 1968, 62 percent , of the total ground level strontium-90 was due to post-1963 tests. HASL estimates that 87 percent of the,fotal reservoir of strontium-90 in 1968 could be attributed / to post-1963 debris. / Concentrations of plutonium-239 in ground level air particulates are presented in Figure 2, The pattern of minimums and maximums is similar to those for Strontium—-90, The same limitations (sample loss and ambiguity) hold as for the nt * It has been suggested that this device exploded at a comparative low altitude (14 km), but that the bulk of the debris was carried to a high altitude by the rising fire-ball, (13) " 3.