Fallout deposition in the Marshall Islands @ H. L. Beck er at. 137 Table 8. Four groups of atolls and islands based on similar cumulative '°’Cs deposition densities (kKBq m~*) from regional fallout. Atolls are listed alphabetically within each group. Atoll group Mean cumulative deposition density of 87Cs (kBq m°) Atolls Range of '*’Cs deposition density (kBq m7’) Southern atolls Ailinglaplap, Arno, Aur, Ebon, Erikub, Jabat, Jaluit, Kili Island, Knox, Lae, Lib Island, Majuro, Maloelap, Mili, Namorik, 1.9 1.3-2.4 Mid-latitude atolls Ailuk, Jemo Island, Kwajalein, Likiep, Mejit 5.1 3.6-7.5 Namu, Ujae Utrik/Taka Northern atolls Island, Ujelang, Wotho, Wotje Utrik, Taka Ailinginae, Bikar, Rongelap Island, Rongerik magnitude of the mean deposition density, the deposition on individual islands could vary considerably. As a meansto verify our estimates of '*’Cs depos- ited from eachtest at each atoll, the deposition estimates at each atoll from each test were each decay-corrected to later years when the inventory of '*’Cs was measured using the effective decay rates discussed earlier. Thus, in making the decay correction, we considered not only radioactive decay but, also, the environmentalloss,1.e., the gradualloss of '°’Cs from the upper layers of soil due to downward migration as a result of long-term precipi- tation. The losses of '’Cs due to both decay and migration are described by the effective decay rate described previously. The sum of the decay-corrected '"Cs depositions from each of the tests was compared with the inventory (corrected for global fallout '°’Cs) at the same atoll measured years after the testing had ceased. A ratio of the estimated total inventory (from summing depositions from each test) to the measured inventory (also reflecting all tests) that was reasonably close to unity was considered as evidence or corroboration that the individual test-specific depositions were reasonable. Where the ratio deviated from unity by more than our estimate of the uncertainty on our sum of the cumulative deposition from all tests, we considered the possibility that some of the measurement data were problematic or that an important source of additional fallout was unaccounted for. To resolve those inconsistencies, we used plots of the geographic pattern of the estimated fallout deposition from each test, as well as from all tests together, supplemented with the patterns of fallout deposition predicted by a meteorological model (NOAAHYSPLIT), to identify where our interpretations of data might have been faulty. These steps were implemented to assist us reformulating individual deposition density estimates with the overall purpose of improving parity between the sum of our test-specific deposition estimates and measurements of total inventory. It is important to note that for manytests andatolls, only sparse historical measurement data were available. 24 82 20-29 54-180 In addition, contemporary measurements, which gener- ally are of high precision, can have limitations in their usefulness since soil disturbance over the intervening years sometimes render them unrepresentative of the original deposition. The iterative technique of comparing fallout measurements with fallout estimates, followed by adjustments based on geographically-based patterns, was used to achieve an improved concordance between the total measured soil '°’Cs inventory and the total estimated '°’Cs deposition. We believe that more reliable estimates of the deposition from individual tests could be made this way compared to relying only on sparse historical measurementdata. The calculated inventory of '*’Cs for each atoll for 1978 and 1991-1993 was comparedwith the average soil inventory measured by Robison et al. (1981) at 12 atolls in 1978 and by Simon and Graham (1997) at 30 atolls in 1993 (see Table 9). Since the depositions occurred at different times, each individual test deposition estimate was decayed (using the effective decay rates given in Table 2) to the time of a relevant soil measurement before summing to make this comparison. The uncertainty in the ratio was calculated from the estimated uncertainty in the individual deposition estimates, in the atoll soil inventory estimates, and in the effective decay. Considering the large uncertainties in each componentof the ratios, the agreementis quite good. For almost all atolls, the final ratio of total estimated '’Cs deposited (from summing test specific estimates) to the contemporary measurements of soil inventory were within the range 0.7 to 1.3, well within the range of uncertainty for the cumulative estimate (Table 9). While most of the individual atolls, ratios are within the range 0.7—1.3, the overall weighted averageratio for both data sets is close to unity. This suggests that our estimated deposition values can be usedto reliably estimate external and internal dosesfor all atolls even though the precision for each atoll individually is not the same. It also suggests that we have not neglected any substantial fallout on anyatoll. The '*’Cs deposition densities (kBq m*) from the Castle (1954) tests Bravo, Romeo, and Yankee, on four