Long-Term and Long-Range Migration of Radioactive Fallout in a Karst System

Abstract

Mountainous areas are often covered by little evolved soils from which deposited radionuclides can potentially leak into the vadose zone. In the Swiss Jura mountains, we observed unusual isotopic ratios of nuclear weapon test (NWTs) fallout with an apparent loss of NWTs plutonium relative to ¹³⁷Cs of Chernobyl origin in thinner soils. Here, we studied the karstic watershed of a vauclusian spring to determine the residence times of plutonium, ²⁴¹Am, and ⁹⁰Sr deposited by global fallout and their respective mobility in carbonaceous soils. The results show that ⁹⁰Sr is washed most efficiently from the watershed with a residence time of several hundred years. The estimated plutonium residence time is more than 10 times higher (in the range of 5000-10,000 years), and the ²⁴¹Am residence time is double that of plutonium. The spring water ²⁴¹Am/²³⁹+²⁴⁰Pu isotopic ratio is lower (0.12 - 0.28) than found in watershed soils (0.382 ± 0.077). Similar differences are found in aquatic mosses (²⁴¹Am/²³⁹+²⁴⁰Pu isotopic ratio 0.05-0.12), which are permanently submerged in spring waters. In contrast to plutonium, ⁹⁰Sr is leached from these mosses with 0.5M HCl, demonstrating that strontium is probably associated with calcium carbonate precipitations on the mosses. The higher plutonium to americium isotopic ratio found in the samples of spring water and mosses at the outlet of the karst shows that plutonium mobility is enhanced.

Evidence of plutonium bioavailability in pristine freshwaters of a karst system of the Swiss Jura Mountains

“…This result demonstrates that the origin of the Pu in the karst system is the fallout of the atmospheric nuclear tests of the sixties. The concentration of 239 Pu in the Venoge spring ranged from 0.43 to 3.44 μBq L -1 (n = 10), similar to concentrations determined previously with alpha-spectrometry (1-7 μBq L -1 , (Froidevaux et al, 2010b). In one case (Venoge water), the 0.5 μm Milligard ® Nominal MCE cartridge cellulose filter was ashed after 300 L of water was filtered through and the Pu measured; less than 10 % of total Pu was found associated with the particulate fraction.…”

“…In general, karst systems are characterized by high water permeability, with underground waters highly enriched in carbonates. Their Pu content comes only from the fallout of the atmospheric nuclear bomb tests of the sixties, as demonstrated in a previous study by an average 238 Pu/ 239+240 Pu activity ratio of 0.031±0.0089 (n = 49) (Froidevaux et al, 2010b).…”

“…This paper reports on the use of the DGT and ultrafiltration techniques to determine the bioavailability of Pu in freshwaters of the karst system located in the Swiss Jura Mountains. A previous study on this system demonstrated a higher mobility of 239+240 Pu compared to 241 Am and 137 Cs (Froidevaux et al, 2010b), suggesting that carbonate-rich vadose waters might contain Pu in a soluble, plutonyl-carbonate form similar to [UO 2 (CO 3 ) n ] m-. To determine the bioavailable Pu, we need to measure the fraction of total Pu that can be taken up by plant through a diffusion process of free and labile metal species (Davison, Nature 1994).…”

Speciation and Bioavailability Measurements of Environmental Plutonium Using Diffusion in Thin Films

“…In addition, molecular isobaric interferences, such as the uranium hydride ( Table 1. 239 Pu concentrations measured in bulk water samples are similar to concentrations previously reported for this aquifer (1-7 µBq L -1 ) 11 . Furthermore, 239 Pu concentrations calculated from DGT measurements are similar within the uncertainties of the measurement.…”

“…Furthermore, we applied large passive DGT samplers of a surface of 105 cm 2 to study the bioavailability of Pu in a karstic spring of the Swiss Jura Mountains (Venoge River) where a significant fraction of Pu was found in the intracellular parts of aquatic mosses in a previous work 11 . Because of the very low level of plutonium present in this pristine environment, accelerator-based mass spectrometry (AMS) techniques available at ETH Zurich were used to measure plutonium isotopes.…”