Uranium series disequilibrium and high thorium and radium enrichments in Karst formations
Von Gunten, H.R.; Surbeck, H.; Rossler, E.
Environmental Science and Technology 30(4): 1268-1274
ISSN/ISBN: 0013-936X DOI: 10.1021/es950473j
We found, in limestone Karst soils of the Jura Mountains and of the mountains in the central part of Switzerland, an enrichment up to a factor 20 of 230Th and 226Ra with respect to the activities of their progenitors, 234U and 238U. Thus, a significant radioactive disequilibrium exists between 238/234U and 230Th and 226Ra. The enrichment of 226Ra leads to locally high concentrations of its decay product, the noble gas 222Rn. We propose continuous chemical weathering of limestone (calcite) fragments within the soil column as a plausible cause for the high 230Th, 226Ra, and 222Rn activities. Uranium, contained within calcite, is released during weathering and migrates as stable uranyl carbonate complexes through the soil column. In contrast, its decay products (230Th and 226Ra) hydrolyze, are strongly sorbed to soil particles, and/or form insoluble compounds that, become more and more enriched in the soil as this process continues in time.