Sources and the global cycle of radium

The principal source of radium in the biosphere is the Earth's crust, from where natural and man made processes transfer this element into the soil, aquatic and atmospheric environments. These processes lead both to the dispersion of radium at the local and global levels and to its enrichment in particular components of the biosphere. In this chapter only the global aspects of this phenomenon are discussed. The purpose here is to give an idea of the magnitude of the flows of 226Ra between compartments. The flow of radium, uranium and thorium from their principal pool in the Earth's crust into the biosphere is determined by hydrothermal and weathering processes. On the basis of current knowledge, only a tentative assessment can be made. The most difficult assessment is probably that of flow from the deeper parts of the crust to the soil layer. There are three main sources of radium in the soil: (a) uranium and thorium present in the minerals that are in the soil and neighbouring rocks; (b) solutions of radium and its parents migrating from deeper layers to the soil; and (c) fallout from the atmosphere of radium and its parents. The average activity concentration of 226Ra in the soil is higher than that of 238U, which reflects a greater mobility of uranium. All human activity contributes only about 1% to the global atmospheric flow of 226Ra and about 5% to the global river flow. this is much less than is necessary to influence the perennial natural fluctuations of 226Ra concentrations, which in pre-industrial precipitation were found to be 28% higher than at present. Atmospheric flows from nuclear power production are about 46 times smaller than from coal burning, but the river flows are similar for both methods. 59 refs, 1 fig., 4 tabs