A combined DGT - DET approach for an in situ investigation of uranium resupply from large soil profiles in a wetland impacted by former mining activities

Abstract

An in situ methodology combining DET and DGT probes was applied in a wetland soil, downstream of a former uranium mine (Rophin), to evaluate metal resupply by calculating the R ratio (R = [U]DGT/[U]pore water) from a high resolution and large (75 cm) soil profile. Our study confirms its applicability in soil layers with varying properties; only soil layers with low water content or coarse texture appear to be limiting factors. For soil profiles, DET provides new insights of the distribution of Uranium as soluble species (free ions, small inorganic complexes, …) along the pore water profile, whereas DGT highlights the presence of other “DGT labile” species. The pairing of DET and DGT, plus the calculation of the R, highlights two U behaviors in combining results from red-ox sensitive elements (Mn, Fe). First, in the organic topsoil layer, an increase in [U]DET and [U]DGT at 3–4 cm reflects the desorption of U probably trapped onto Fe- and Mn-oxohydroxides in a DGT-labile form. However, the resupply from soil to pore water is close to a diffusion only case (R < 0.2) meaning that a portion of U is certainly tightly bound by OM in soil as non-labile species. Second, a peak in [U]DGT perfectly corresponding to the former mine deposit layer signifies the presence of U under DGT-labile species. Moreover, a maximum R value of 0.87 demonstrates the near complete resupply of U from a labile fraction in this layer, as opposed to other elements like Pb.