Table 2— COMPARISON OF WORLD-WIDE DISTRIBUTION OF Sr™ Cs/37| AND Pu®® FROM NUCLEAR DETONATIONS* Mid-1957 Sy? Region Northern USA North temperate latitudes South temperate latitudes Rest of world mc/sq mile cgi! Pu2s? me/sq mile mc/sq mile World average 35 19 5-6 3-4 8 46 25 7 5 2.1 1.2 0.3 0.2 Total surface deposition 1.64 MC 2,1 MC 0.10 MC Stratospheric reservoir 1.10 MC 1.4 MC 0.06 MC 10 0.45 * Assuming no fractionation. simplified. Some fractionation is indicated by air sampling data and once fission products are suspended in the troposphere (either directly from the detonation or from stratospheric leakage, regardless of mechanism) meteorological conditions play a major role in their sur- face distribution. Libby has stressed the importance of rainfall, snow, fog, and mist.>»” Within any major area fluctuations in levels of surface deposition may occur which correlate with local meteorological conditions. Machtahas guessed that areas as large as milksheds may not have more than 2 to 3 times the average deposition for the latitude. He points out, however, that desert areas where there is practically no rainfall may have almost zero fallout. (b) Future Levels (Assuming No More Tests). Fallout of Sr® and other long-lived radionuclides from the stratospheric reservoir will continue even if weapons tests are stopped. Whether the integrated surface deposition levels continue to build up will depend on whether the rate of stratospheric fallout more than compensates for the rate of decay of material already on the ground. From the surface deposition levels in Table 2 and the value of 1.1 + 0.9 megacuries of Sr®™for the Stratospheric reservoir, estimation of future deposition levels, assuming no more ‘weaponstests, is possible. If M(t) is the surface deposition level and Q(t) is the stratospheric storage in millicuries per square mile at any time, the rate of change of the surface deposition level is: dM(t) at _ -AM(t) + kQ(t) where A is the radioactive decay constant of Sr®, and k is the stratospheric fallout rate constant (assumed to be first order). If AM(t) = kQ(t), dM/dt is zero. In this case, additional stratospheric fallout just compensates for radioactive decay, and M(t) does not change. Such an equilibrium state is transitory, since Q(t) is constantly decreasing (both by decay and by fallout). Loss by radioactive decay in M(t), therefore, soon exceeds gain from Q(t), and M(t) falls. If AM, is greater than kQ, (where My and Q) are the concentrations at t = 0, the time of cessation of tests), the latter situation already exists and the ground level begins to fall when testing stops. Only if AM) is less than kQ, will additional fallout from the stratosphere exceed the decay of the ground contamination and the surface deposition level continue to rise. If the mean times of decay and fallout are 40 and 10 years, respectively, Q(t) must be at least ¥. M(t) for surface deposition to increase. Future levels, in the event of no more testing, can be estimated if it is assumed that fallout in the future will have the same degree of nonuniformity as in the past. In this case, the effective stratospheric storage (Q(t).) for a given area is related to the average stratospheric Storage (Q(t)ay) by the equation: Q(te = a Q(t) ay 288