Biogeochemical Cycles of Long Lived Radionuclides

The radioactive debris injected into the stratosphere from nuclear weapons tests takes years to deposit, during which time the shorter-lived radionuclides largely disappear through substantially radioactive decay and gravitational settling, whereas the longer-lived radionuclides (such as 90Sr (strontium-90, t1/2 = 28.8 years), 137Cs (cesium-137, t1/2 = 30 years), and the plutonium isotopes 238Pu (t1/2 = 87.7 years), 239Pu (t1/2 = 2.4 x 104 years), and 240Pu (t1/2 = 6.6 x 103 years)) remain in the atmosphere. The study oflong-lived radionuclides has yielded useful information for understanding global biogeochemical cycling and various physical, chemical, and biological processes in terrestrial and aquatic ecosystems. 137Cs and Pu isotopes are chemically reactive, and Sr is less reactive. Generally, 90Sr migrates more rapidly than 137Cs in the soil layer. In other words, in the soil Sr is more mobile than 137Cs, which is strongly adsorbed on clay and is essentially nonexchangeable. But the migration of 137Cs depends largely on soil characteristics. Hence, unexpectedly high levels of 137Cs in milk, vegetation, and animal tissues were found in a region of the southeastern United States characterized by very sandy soils in which 137Cs could rapidly migrate into vegetation through uptake and eventually to the entire food chain. This is in sharp contrast to the behavior of 137Cs in most ecosystems in the United States, where 137Cs is relatively quickly and nearly irreversibly bound to clay in soil. The uptake of 137Cs from a sandy soil is about five times higher than from a clay soil; for a loam soil, this factor is about 2. High organic matter content in soil is also believed to play a major role for the mobility of cesium. Another important factor that influences uptake is pH. Uptake increases with decreasing pH values.

The weapons testing fallout of 90Sr and 137Cs that has affected terrestrial and aquatic ecosystems has been reported extensively in the literature. A number of studies on 90Sr, 137Cs, and Pu isotopes have shown the continually downward movement of these radionuclides into the soil. These radionuclides may remain in the soil for many years, resulting in uptake by plants into vegetation and the entire food chain through biogeochemical cycles, as mentioned earlier, in the southeastern United States. As another example, the island environment of the atolls in the Marshall Islands represents a unique ecosystem where radionuclides that were introduced between 1946 and 1958 have had nearly 50 years to equilibrate. Bikini and Enewetak atolls were the sites of 66 atmospheric nuclear weapons tests. These atolls are composed of coral limestone. The composition of the atoll soil produces dramatic differences in the uptake of 137Cs and 90Sr at the Marshall Islands compared with the uptake rates discussed in the literature, which are based primarily on silica-clay-type soils. For instance, the concentration ratio (CR), defined as the activity concentration of the radionuclide per gram of wet plant soil divided by the activity concentration per gram of dry soil, is about 0.1 for 137Cs, and about 1.0 for 90Sr in silicate soils. However, in the coral soils in the Marshall Islands the CR for 137Cs is about 5, and that for Sr is about 0.0001. This is in stark contrast to the very different CRs observed for 137Cs and 90Sr in different soil systems. Also, the atoll ecosystem is ideal for evaluating the root uptake of 239 + 2 0Pu by plants. It was found that the CRs for plutonium generally agreed with pot culture studies in glasshouses. The general magnitude of uptake of plutonium seems to be about the same over a wide range of soil types, with the coral soils being at one extreme with high pH and nearly pure CaCO3 plus organic material. It indicates the importance of a fundamental understanding of biogeo-chemical cycles and pathways of radionuclides in different ecosystems when the prediction of impacts of radionuclide contamination is made.

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