Comparison of 90Sr/137Cs activity ratios in the soil of fallout plumes from aboveground nuclear and thermonuclear tests at the Semipalatinsk Test Site

Nuclear weapons testing leaves behind radioactive contamination as a long-lasting legacy and public health hazard at the Semipalatinsk Test Site (STS) and the ⁹⁰Sr and ¹³⁷Cs radionuclides are fission products with relatively high bioavailability due to the similarity of strontium to calcium and cesium to potassium. This study investigated ⁹⁰Sr and ¹³⁷Cs activity concentrations and ⁹⁰Sr/¹³⁷Cs ratios in topsoil (0–5 cm) and soil particle-size fractions collected in the fallout plumes from the 38 kt aboveground nuclear test conducted on September 24, 1951 (the Southern plume) and the 400 kt first thermonuclear test on August 12, 1953 (the Southeastern plume) at the center of the Experimental Field test site of the STS. From our findings, activity levels of radioactive ¹³⁷Cs contamination were comparable in the two plumes, but the Southeastern plume had higher ⁹⁰Sr levels. The estimated ⁹⁰Sr/¹³⁷Cs ratios were between 0.4 and 1.7 for the Southern plume, while the ratios of thermonuclear test's plume ranged between 1.1 and 5.4, which are assumed to be due to the different types and yields of the nuclear charges. For the Southeastern plume, the values of the ⁹⁰Sr/¹³⁷Cs ratios in soil particle-size fractions reduce as the size of the fractions decreased from 1000 to 40 μm. High activity concentration ratios of up to 6.2 were observed for fractions ranging from 250 to 1000 μm. In the Southern plume, radionuclides were concentrated in the 250-100 μm fraction, without variability of the radionuclide ratios based on particle-size fractions. The maximum ⁹⁰Sr/¹³⁷Cs ratio for the Southern plume did not exceed 3, and was less than 1 for finer fractions. As a result, the distribution of radionuclides by particle-sized fractions of soils in combination with other precision diagnostic methods of radioactive particles can help to identify sources and nature of radioactive fallout and to assess the degree of transformation of radioactive fallout over time, as well as the long-term effects of radioactive contamination on various environmental components.