An evidence of chemically and physically mediated migration of 238U and its daughter isotopes in the vicinity of a former uranium mine.

The present study reports the evidence of a radioactive contamination in a wetland located downstream from a former French U mine in Brittany. This situation is demonstrated according to the measurements of gamma dose rates and activity ratios of 238U and 232Th-decay series nuclides, which give the justification regarding the accumulation of significant amounts of 238U, 230Th and 226Ra in this wetland. The dose rate map highlights an increase of radiation level along the former mine water pathway compared to the background value, with a maximum value of 1500 nSv.h-1 reached in the wetland. Activities of 238U, 230Th and 226Ra and 232Th/238U ratios measured in surface wetland soils are significantly higher than the geochemical background. 230Th/238U ratios less than unity suggest a preferential accumulation of U in the wetland, compared to its daughter isotopes. Moreover, the loss of 226Ra compared to 230Th raises its higher mobility compared to its parent isotope. In far-field sediments, 226Ra/238U ratio of 1.76 implies a different geochemical behavior of U, which could be explained by the occurrence of mobile U species. The results suggest that contamination of wetland soils and far-field sediments could result from discharges of underground mine waters.

Derivation of ecotoxicity thresholds for uranium.

Assessment of the risk of impact from most radionuclides is based on the total radiological dose rate to the organism of concern. However, for uranium (U) there can be greater risk from chemical toxicity than radiological toxicity (depending on the isotopic composition). Chemical ecotoxicity of U is dependent on several environmental parameters. The most important are carbonate content, because of the formation of soluble carbonate complexes, and divalent cation content (Ca++ and Mg++), because of their competitive interaction with the uranyl ion (UO2++). This study summarizes the literature available to set PNECs (predicted no-effect concentrations) for chemical toxicity of U to non-human biota. The corresponding radiological doses were estimated, and as expected chemical toxicity proved to be the greater concern. There were limited data from some types of biota; however, PNECs for the types of biota of interest were as follows: terrestrial plants--250 mg U kg(-1) dry soil; other soil biota--100 mg U kg(-1) dry soil; freshwater plants--0.005 mg U L(-1) water; freshwater invertebrates--0.005 mg U L(-1) water; freshwater benthos--100 mg U kg(-1) dry sediment; freshwater fish at water hardnesses of: <10 mg CaCO3 L(-1) (very soft water)--0.4 mg U L(-1) water; 10-100 mg CaCO3 L(-1) (soft water)--2.8 mg U L(-1) water; and >100 mg CaCO3 L(-1) (hard water)--23 mg U L(-1) water; or as a function of hardness--0.26 (hardness as mg CaCO3 L(-1); mammals--0.1 mg U kg(-1) body weight d(-1).