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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 238 U and 232 Th-decay series nuclides, which give the justification regarding the accumulation of significant amounts of 238 U, 230 Th and 226 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 -1 reached in the wetland. Activities of 238 U, 230 Th and 226 Ra and 232 Th/ 238 U ratios measured in surface wetland soils are significantly higher than the geochemical background. 230 Th/ 238 U ratios less than unity suggest a preferential accumulation of U in the wetland, compared to its daughter isotopes. Moreover, the loss of 226 Ra compared to 230 Th raises its higher mobility compared to its parent isotope. In far-field sediments, 226 Ra/ 238 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., (Copyright © 2018 Elsevier Ltd. All rights reserved.)

Knowledge of sedimentation rates in lakes is required to understand and quantify the geochemical processes involved in scavenging and remobilization of contaminants at the Sediment-Water Interface (SWI). The well-known 210 Pb excess ( 210 Pb ex ) method cannot be used for quantifying sedimentation rates in uranium-enriched catchments, as large amounts of 210 Pb produced by weathering and human activities may dilute the atmospheric 210 Pb. As an alternative dating method in these cases, we propose an original method based on 232 Th decay series nuclides. This study focuses on an artificial lake located in a granitic catchment downstream from a former uranium mine site. The exponential decay of 228 Ra excess ( 228 Ra ex ) with depth in two long cores yields sedimentation rates of 2.4 and 5.2 cm yr -1 respectively. These sedimentation rates lead to the attribution of the 137 Cs activity peak observed at depth to the Chernobyl fallout event of 1986. The 228 Ra ex method was also applied to two short cores which did not display the 137 Cs peak, and mean sedimentation rates of 2.1 and 4.0 cm y -1 were deduced. The proposed method may replace the classical radiochronological methods ( 210 Pb ex , 137 Cs) to determine sedimentation rates in granitic catchments., (Copyright © 2016 Elsevier Ltd. All rights reserved.)