Uranium series disequilibrium and high thorium and radium enrichments in Karst formations

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 Th-230 and Ra-226 With respect to the activities of their progenitors, U-234 and U-238. Thus, a significant radioactive disequilibrium exists between U-238/234 and Th-230 and Ra-226. The enrichment of Ra-226 leads to locally high concentrations of its decay product, the noble gas Rn-222. We propose continuous chemical weathering of limestone (calcite) fragments within the soil column as a plausible cause for the high Th-230, Ra-226, and Rn-222 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 (Th-230 and Ra-226) 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.