where M(t) is the observed ground concentration in the area in question, and M(t)ay is the averaged world-wide ground concentration. On the basis of this assumption, the soil levels increase everywhere by the same ratio and reach a maximum about 1963, which is some 10 per cent higher than present levels. Assuming uniform stratospheric fallout, some areas do not increase since the additional stratospheric fallout is insufficient to compensate for radioactive decay. The time of maximum ground concentration (where it does occur) varies also with location, being about 1966 in the south temperate latitudes and 1969 elsewhere. Neither method of estimation is strictly correct. The assumption of uniform fallout may underestimate build-up in the northern latitudes, and the assumption of nonuniformity of future fallout according to the past may tend to overestimate build-up in those areas where some of the material deposited in the past came from tropospheric fallout. As stated by Machta,* it is hoped that the truth lies somewhere in between. It must also be kept in mind that the stratospheric reservoir may well be 2.4 megacuries as estimated by Libby.” Future Cs!*" levels, assuming no fission product fractionation and no moretests, will be about 1.3 times higher than the corresponding Sr*® levels since their radiological half lives are essentially the same. Pu?** levels will continue to rise for several years becauseofits 24,000-year half life. In this case, AM) will be less than kQ, until the stratospheric reservoir is essentially depleted. However, surface deposition levels will not increase more than 0.6, which is the ratio of the present total surface deposition to the estimated stratospheric reservoir. Table 3— PREDICTED AVERAGE MAXIMUM SURFACE DEPOSITION LEVELS OF Sr”, csi? AND pu? (ASSUMING NO MORE WEAPONS TESTS AFTER MID-1957) Region Northern USA mc/sq mile* mc/sq milet North temperate latitudes 21 39 51 3.3 South temperate latitudes 6 8 0.5 9 12 0.8 Rest of world World average | me/sq mile* 4 27 5 2.0 0.3 *Maximum will be reached in about 1965, * Maximum will be reached essentially in about 30 years. Predicted average maximum surface deposition levels of Sr*°, Cs!57, and Pu?®® (assuming nonuniform fallout and cessation of tests) are given in Table 3. Surface deposition levels of other biologically significant isotopes, which all have short half lives comparedto the stratospheric storage time and for which AM) is already greater than kQ), will begin decreasing immediately when weapons tests are stopped. (c) Future Levels (With Continued Testing). If weapons tests continue at a constant rate (in terms of fission yield), the decay of radionuclides in the biosphere will eventually equal the rate of production, and continued testing will result in no further increase in deposition levels. At the present rate of testing (assumed to be 10 megatonsof fission per year for the past 5 years), equilibrium Sr® and Cs'*’ levels will be reached in about 100 years. Isotopes with shorter half lives will reach equilibrium sooner. Py? obviously will continue to increase essentially in proportion to its total production. Campbell'« and Stewart et al.4 have estimated surface deposition levels of Sr*® at equilib- rium with a uniform test rate, and their calculations suggest levels about 30 times the present values. Their equations are derived, however, from stratospheric fallout and apply to ground levels due to the stratospheric component only. Libby? estimated surface build-up on the basis of total levels on the ground at t = 5 years and predicted equilibrium levels 11 times the present values. His calculations have been checked by Neuman!® and others. Libby also assumed that about 30 per cent of the Sr®™(over 289