An evidence of chemically and physically mediated migration of U 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 U and Th-decay series nuclides, which give the justification regarding the accumulation of significant amounts of U, Th and Ra 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 reached in the wetland. Activities of U, Th and Ra and Th/U ratios measured in surface wetland soils are significantly higher than the geochemical background. Th/U ratios less than unity suggest a preferential accumulation of U in the wetland, compared to its daughter isotopes. Moreover, the loss of Ra compared to Th raises its higher mobility compared to its parent isotope. In far-field sediments, Ra/U 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.