G. FAURE, J.A. SIPP, and G. LEE, Department of Geological Sciences, Ohio State University, Columbus, Ohio 43210
Cesium -137 is a product of nuclear fission of uranium-235. It has a half-life of 30.1 years, which means that more than 150 years must elapse before the decay rate of this radionuclide decreases to about 3 percent of its initial value. Cesium-137 has been widely dispersed in the Northern Hemisphere as a result of testing of nuclear weapons and processing of spent nuclear fuel used in uranium-fission reactors and because of the accidental explosion of a reactor in Chernobyl, Ukraine, on 26 April 1986 (Bunzl et al. 1995; He, Walling, and Owens 1996; Faure in press).
The cesium-137 that was released into the atmosphere by the reactor accident in Chernobyl has contaminated the Northern Hemisphere of the Earth including the United States and Canada, which received amounts of this nuclide equivalent to decay rates of 2.8 x 1015 Becquerels (Bq) and 2.5 x 1015 Bq, respectively (Anspaugh, Catlin, and Goldman 1988). One Bq is the amount of a radionuclide having a decay rate of 1 disintegration per second.
This study was undertaken to determine whether cesium-137 is detectable in soil of the ice-free valleys of southern Victoria Land. For this purpose, soil samples were collected in December 1994 at one location each in the Beacon Valley, in Wright Valley, and in Taylor Valley. At these locations, incremental layers of soil were removed from about 1,500 square centimeters of surface area and the depth intervals were later determined in the laboratory from the weights and measured densities of the samples shown in the figure.
Portions of the soil samples were analyzed by Teledyne Brown Engineering Environmental Services using gamma-ray spectrometry based on 12-hour counts of 10-gram samples. The results pertain not only to cesium-137, but also record the presence of certain radioactive atoms of actinium, thallium, bismuth, and lead that form naturally as decay products of uranium and thorium in the rocks of southern Victoria Land and elsewhere. In addition, potassium-40 (a long-lived, naturally occurring radioactive isotope of potassium) was detected in all soil samples recovered from the three sites. All of the observed counting rates were corrected for the efficiency of the detector and are expressed in units of picocuries per gram of sample, where 1 picocurie equals 0.037 Bq.
We present here only the results for the soil profile in Beacon Valley taken on the valley floor at about 77°52'S and 160°30'E at an elevation of 1,440 meters above sea level. The profile consists of 10 samples extending from the surface to a depth of 14.4 centimeters (about 5.7 inches). The pebble-sized rock clasts of the lag-gravel present at this site were removed before soil samples were taken.
The results in the figure indicate that the density of the soil increases at a depth of 9.64 centimeters from 1.49±.07 to 1.78±.05 grams per cubic centimeter. The decay rates of cesium-137, however, in the soil samples from Beacon Valley and from the other sites in southern Victoria Land are all less than the limit of detection of this nuclide, which varies from less than 0.05 to less than 0.08 picocuries per gram. Therefore, cesium-137 was not detected at any of the three locations in the ice-free valleys.
A decay rate of less than 0.08 picocuries of cesium-137 means that 1 gram of soil contains fewer than 4 million atoms of this radionuclide and that none may be present. In marked contrast, a soil sample from a prairie on the Marion campus of Ohio State University was found to contain 228 picocuries of cesium-137 per gram or almost 300 times more than the highest possible concentration recorded in the Beacon Valley samples.
The figure also demonstrates that the decay rate of potassium-40 in the layer of dense soil below a depth of 9.64 centimeters is 35 percent higher than that of the overlying soil. The increase in the potassium concentration of the soil at this site in Beacon Valley reveals the presence of stratification even though the grain-size distribution indicates that the entire soil is of glacial origin.
This study was supported by National Science Foundation grant OPP 93-16310.
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Bunzl, K., H. Kofuji, W. Schimmack, A. Tsummura, K. Ueno, and M. Yamamoto. 1995. Residence times of global weapons testing fallout 237Np in a grassland soil compared to 239+240Pu, 241Am, and 137Cs. Health Physics , 68(1), 89-93.
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He, Q., D.E. Walling, and P.N. Owens. 1996. Interpreting the 137Cs profiles observed in several small lakes and reservoirs in southern England. Chemical Geology , 129, 115-131.