Origin and stability of uranium accumulation-layers in an Alpine histosol

Jasquelin Peña; Marietta Straub; Virginie Flury; Eymerick Loup; José Corcho; Philipp Steinmann; François Bochud; Pascal Froidevaux

doi:10.1016/j.scitotenv.2020.138368

Origin and stability of uranium accumulation-layers in an Alpine histosol

Sci Total Environ. 2020 Jul 20:727:138368. doi: 10.1016/j.scitotenv.2020.138368. Epub 2020 Apr 8.

Authors

Jasquelin Peña¹, Marietta Straub², Virginie Flury¹, Eymerick Loup¹, José Corcho², Philipp Steinmann³, François Bochud², Pascal Froidevaux⁴

Affiliations

¹ Institute of Earth Surface Dynamics, Faculty of Geosciences and Environment, University of Lausanne, Lausanne, Switzerland.
² Institute of Radiation Physics, Lausanne University Hospital and University of Lausanne, Lausanne, Switzerland.
³ Division of Radiation Protection, Federal Office of Public Health, Berne, Switzerland.
⁴ Institute of Radiation Physics, Lausanne University Hospital and University of Lausanne, Lausanne, Switzerland. Electronic address: pascal.froidevaux@chuv.ch.

PMID: 32334206
DOI: 10.1016/j.scitotenv.2020.138368

Abstract

Uranium (U) accumulation in organic soils is a common phenomenon that can lead to high U concentration in montane wetlands. The stability of the immobilized U in natural wetlands following redox fluctuations and re-oxidation events, however, is not currently known. In this study, we investigated a saturated histosol that had accumulated up to 6000 ppm of U at 30 cm below ground level (bgl). Uranium in the waters feeding the wetland originates from the weathering of surrounding gneiss rocks, a process releasing trace amounts (<3 ppb) of soluble U into nearby streams. Redox oscillations in the first 20 cm bgl led to the accumulation of U, Ca, S in low permeability layers at 30 and 45 cm bgl. XRF measurements along the core showed that U strongly correlates with sulfur (S) and calcium (Ca), but not iron (Fe). We tested the stability of uranium in the histosol over a nine-month laboratory amendment of a large core of the histosol (∅ 30 cm; length 55 cm) with up to 500 ppm nitrate. Nitrate addition was followed by complete nitrate reduction and re-generation of oxidizing E_h conditions in the top 25 cm of the soil without U release to the soil pore waters above background levels (1-2 ppb). Our results demonstrate that, fast reduction of nitrate, sulfate, and Fe(III) occur in the soil without U release. The remarkable stability of sorbed U in the histosol may result from buffering by sulfide and S_n° and/or strong U(IV)-OM or U(VI)-OM enhanced by organic S moieties or bridging complexation by Ca. That U in the soil was immobile under nitrate addition for up to 9 months can inform remediation strategies based on the use of artificial wetlands to limit U mobility in contaminated sites.

Keywords: Groundwater quality; Nitrate amendment; Organic matter; Redox cycling; U mobility.