Evaluation of the application of Diffusive Gradients in Thin Films (DGT) technique in water, soils and sediments as a monitoring tool in uranium mining environments

English Abstract:

In France, uranium mining activities in 1945-2001 led to the production of 76,000 t of uranium. To date, all these sites have been rehabilitated and a site-specific survey is performed by AREVA, local and public authorities. AREVA has undertaken R&D studies to perform hydro-geochemical modelling of these storage sites. Although the radiological impact of uranium is determined by the total concentration; the aquatic chemistry, its toxicity and bioavailability are determined by chemical speciation. The technique of Diffusive Gradients in Thin Films (DGT) is an in situ monitoring tool for the bioavailable fraction, as it provides the time-averaged concentrations of labile metal species in solution. The technique is also applied to soils and sediments, revealing high resolution porewater profiles and remobilization rates from the solid phase. The evaluation of the technique was performed in laboratory and in field conditions. The mining sites in France with different geochemical characteristics and water treatment technologies were investigated. DGTs were deployed in stream water upstream and downstream of the mining sites; within the various stages of the water treatment, in a wetland and in lake sediments. A device displayed high resolution profiles in sediments and soils up to a 1.5 m depth. The DGT technique was compared to filtration/ultrafiltration techniques and geochemical speciation. In the laboratory, different binding phase for U (Chelex^®-100, Metsorb™, Diphonix®) in the DGT devices were evaluated. The different resins were compared across a wide pH (3‑9) and ionic strength (0.001‑0.7 M NaNO₃) range. Possible interferences with Ca²⁺ (up to 12.5 mM), PO₄^3- (up to 0.05 mM), SO₄^2- (up to 2.1 mM) and HCO₃^- (up to 8.2 mM) on U-DGT uptake were investigated. The laboratory and field studies along with speciation modelling allows better understanding of the applicability of DGT as a monitoring and speciation tool in mining environments.