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.

Estimation of sedimentation rates based on the excess of radium 228 in granitic reservoir sediments.

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 210Pb excess (210Pbex) method cannot be used for quantifying sedimentation rates in uranium-enriched catchments, as large amounts of 210Pb produced by weathering and human activities may dilute the atmospheric 210Pb. As an alternative dating method in these cases, we propose an original method based on 232Th 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 228Ra excess (228Raex) 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 137Cs activity peak observed at depth to the Chernobyl fallout event of 1986. The 228Raex method was also applied to two short cores which did not display the 137Cs 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 (210Pbex, 137Cs) to determine sedimentation rates in granitic catchments.