The presence of artificial radioisotopes of plutonium, americium and strontium is mainly due to the atmospheric tests of nuclear bombs before 1964. Present day inventories of these radionuclides in soils of the Jura Mountains (Switzerland) often are less than expected. Apparently, there has been leakage from the soils (especially the thin soils) into the karst of this region of carbonate rocks.
With the goal to assess this possible long-range migration in a natural setting we analyzed the radioisotopes
- in upland soils (soil profiles and soil solution profiles near Col de Mollendruz, Switzerland, 1200 a.s.l.),
- in a karstic source at a distance of ca. 5 km (water and aquatic mosses, 660 m a.s.l.), and
- in cave deposits from the same area.
The mean soil inventories (0-25 cm) for thick soils are 1.3 kBq/m2 and 0.12 kBq/m2 for 90Sr and Pu, respectively. The radioisotopes have significantly diffused towards deeper soil layers (40 cm depth or more) and they are also present in measurable amounts in the soil solution (< 0.22 μm, i.e. dissolved and colloidal). These findings highlight the potential of long-range transport in colloidal form. The calculated distribution coefficients Kd are on the order of 10'000; 3'000; and 1'500 for Pu, Am, and Sr, respectively.
The radioisotopes appear in the waters of the karstic source and in aquatic mosses collected close to the source. The plutonium activity found in the filtered (<0.45 μm) source water is 4 and 22 μBq l-1 (U95 = ±25 %) at high-stand and lowstand, respectively. The Sr-90 activity of these waters is approximately 1000 times higher. In the mosses we find up to 20 mBq/g or more of Sr-90. However, most of this activity is associated with carbonates, which have precipitated on the mosses after degassing of CO2 from the source waters. The Pu activities of the moss samples vary between 0.1-0.2 mBq/g.
Taken together our results demonstrate the long-range transport (km-scale) of Pu, Am, and 90Sr in a natural karstic environment with no radioisotope contamination other than "global fallout".