Uranium Lability in soils downstream a former mine (France) using a coupled DGT/DET approach

International audienceBackground: Uranium mining activities expose uranium ore and mine tailings to surface environment where the release of radionuclides is facilitated by weathering at rates exceeding those typically found in nature. In Rophin former mine site (Puy-de-Dôme, France), the measurement of radioactivity in a wetland located downstream the site suggests a contamination of the soil surface. Material and methods: A combination of DGT and DET samplers (Chelex® resin) were deployed in situ in soil in order to record Uranium vertical depth profile. Thus, a specific PVC plate was in purpose designed allowing the use of 5 DGT/DETpairs, in back to back position, over a depth of 75 cm. With this system, soil reactivity coefficient (R), defined as [U]DGT/ [U]DET ratio, can be calculated all along the profile. R values ranges from 0 to 1 for inert to reactive soils respectively. In addition, soils horizons were sampled (drill core) and characterized for water and organic matter contents, CHNS, grain size, major and trace elements (Fe, Mn, Ba, Pb) composition.Results: Soil cores consist of three horizons (1-3): 1 [0-15 cm] rich in organic matter (OM) with a high content of water typical of wetlands, 2 [15-50 cm] mix of silty loam and sand characteristic of a discharge from a settling basin used during ore washing process and 3 [50-70 cm], mainly organic, identified as the original soil existing before mining exploitation. Uranium concentrations, measured in dry soils, decrease from 1500 mg/kg in horizons 1 and 2 to 50 mg/kg in horizon 3. In horizon 1 (soil surface), R values are low (R = 0.2) indicating an immobilization of Uranium by OM present at high content. On the contrary, in horizon 2, R values are close to 1 suggesting a high lability of Uranium. Conclusions: Combining DGT/DET samplers for in situ measurement provides new insight on Uranium lability in wetland under influence of former mining activities. R ratio values suggest a high availability of U in soil horizon deposited by mining activities. This in-situ methodology coupled with 2D DIFS-modeling might enable to calculate representative kinetics parameters of Uranium resupplying process