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

Publication date: December 2018

Source: Journal of Environmental Radioactivity, Volume 195

Author(s): Arnaud Mangeret, Pascale Blanchart, Gilles Alcalde, Xavier Amet, Charlotte Cazala, Marie-Odile Gallerand

Abstract

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.

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