Early diagenesis of radium 226 and radium 228 in lacustrine sediments influenced by former mining sites

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Highlights

  • The early diagenesis of long-lived radium isotopes 226Ra and 228Ra was evaluated in U-mining impacted granitic lake sediments.

  • Ra activities in sediment pore waters were measured for the first time using very low background Germanium well detectors.

  • 226Ra and 228Ra early diagenesis seem to be closely related to the reductive dissolution of iron oxyhydroxides.

  • Calculated diffusive fluxes of Ra are significantly lower than those previously reported in Ra-enriched sediments.

Abstract

Radium is a naturally occurring radioactive element commonly found at low levels in natural systems such as lacustrine or marine sediments. Anthropogenic activities including former uranium mining activities can lead to the dissemination of radium isotopes having high radiological toxicities, which potentially threaten the safety of nearby environments. Although radium mobility in oxidized environments is known to be largely governed by sorption/desorption onto Fe and Mn oxyhydroxides and coprecipitation with sulfate minerals (e.g. barite), little is known regarding its behavior under reducing conditions, which are the conditions typically encountered in organic-rich systems such as wetlands and lake sediments.

The present study aims at understanding the behavior of long-lived radium isotopes (226Ra and 228Ra), during early diagenesis of lake sediments contaminated by former uranium mining activities. Solid and pore water concentrations of 226Ra and 228Ra were determined using ultra low background gamma spectrometry, which allowed improvement of detection limits and measurement accuracy.

This study shows that the downcore distribution of radium isotopes is closely related to the reductive dissolution of iron and manganese oxyhydroxides below the sediment-water interface. The resulting diffusive fluxes of 226Ra and 228Ra (4.1 10−25 and 4.7 10−28 mol cm−2.s−1) are however significantly lower than other radium-impacted environments, such as uranium mill tailings pond and phosphate industry-impacted sediments, and are similar to those reported for natural marine environments. Hence, in the reduced lake sediments of Saint-Clement, the major fraction of radium is trapped by the solid phase, while early diagenesis only induces a slight mobility of this radioelement.

Introduction

Uranium extraction activities generate high amounts of potentially hazardous byproducts including waste rocks, mill tailings, and can also impact lands, river and lake sediments (Abdelouas, 2006; Morin et al., 2016), which can result in their enrichment in natural radionuclides such as 226Ra (Carvalho et al., 2014; Reyss et al., 2016). Due to the radiotoxicity of this radionuclide, contaminated materials must therefore be appropriately managed in the long term in order to minimize its potential migration to and accumulation in human and ecological targets.

To date, the knowledge on radium mobility and dynamics in low-temperature natural systems is firstly derived from studies conducted in the marine environment. Radium isotopes are indeed frequently used as proxies for the determination of groundwater discharge fluxes into the coastal ocean (Moore, 1996; Krest and Harvey, 2003) and transport of pore water solutes (Sun and Torgersen, 2001). In these environments, the distribution of radium is known to be indirectly controlled by redox processes. For example, radium-bearing phases can be sensitive to redox variations, especially iron and manganese (oxyhydr)oxides and oxides, respectively (Gao et al., 2010; Nirdosh et al., 1984, 1990) and sulfate minerals such as barite (Doerner and Hoskins, 1925; Henkel et al., 2012).

The current knowledge of radium geochemistry in natural systems is also derived from studies conducted in continental systems (Greeman et al., 1999; Rihs et al., 2011) or in radium-enriched waste such as uranium mill tailings and adjacent surface waters and ponds (Sebesta et al., 1981; Wiles, 1983; Landa and Gray, 1995; Peacey et al., 2002; Martin et al., 2003). Although a large number of studies have addressed the radium behavior in marine and continental systems, less is known about the processes governing the fate of radium isotopes from reducing environments, except for the study of Martin et al. (2003) conducted in a tailing pond. Notably, the influence of early diagenesis processes on the stability of radium-bearing species in reduced lake sediments is poorly documented. This knowledge is however crucial to choose the appropriate strategy to manage radium-enriched sediments.

The lack of data is notably significant regarding processes that could control radium mobility in anaerobic systems. This gap is partly related to the challenges encountered for i) sampling significant volumes of anaerobic sediment pore waters without modifying radium distribution and ii) measuring low radium isotopes activities in these pore waters. As an example, the study of Gao et al. (2010) leads to the quantification of low 226Ra fluxes from sediments to the overlying waters based on the use of Diffusive Gradient in Thin Films (DGT) techniques and concludes that labile 226Ra can be related to the reductive dissolution of iron and manganese (oxyhydr)oxides without clearly demonstrating the close association between Ra and Fe association in the solid phase.

The present study aims at providing relevant clues on the geochemical processes controlling the radium distribution in the sedimentary column of a lake. The behavior of 226Ra is compared with analogous solutes such as 228Ra and barium. The obtained results reveal important geochemical factors that may be considered for better predicting the post-depositional mobility of radium isotopes in impacted lacustrine systems, as well as, by extension, in underwater radioactive waste storage.

Section snippets

Site description and sampling procedures for sediment and pore water

The lake Saint-Clément is an artificial reservoir created in 1931 for electricity generation. This lake is located at nearly 20 km downstream from the former uranium mining site of Bois-Noirs-Limouzat and its tailing storage in the Massif Central (Fig. 1).

A detailed description of the site can be found in Morin et al. (2016), Reyss et al. (2016) and Stetten et al. (2018). Lake sediment cores were sampled within PVC liners using a 90 mm diameter Uwitec® hand corer. Sediments cores were collected

Solid phase Ra concentration profiles

The activities of the 238U-serie nuclides in the sediment solid phase are reported in Fig. 2 and Table 2 for core 2016C2I and are roughly similar to those measured in a previous study on the same lake (Reyss et al., 2016). The activities of 238U, 226Ra and 210Pb are fairly constant until approximately 90 cm depth, reflecting the deposition of the same sediment type until this depth. From 90 to 120 cm depth, the 238U, 226Ra and 210Pb activities increase sharply up to 3862, 1154 and 1245 Bq.kg−1,

Conclusions

This study reported original data regarding the effects of early diagenesis on the mobility of radium isotopes in the sediments of the lake Saint-Clément, located downstream from a former uranium mining site. Determination of the radium activities was successfully performed for the first time in small pore water volumes, carefully sampled under inert atmosphere, using well-type Germanium detectors, which is challenging.

Our results suggest that the Ra released in sediment pore water of the lake

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

We thank Gilles Alcalde, Anthony Julien, Olivier Diez and Marine Garcia for their help in field sampling and chemical analyses, respectively. This work was supported by the French Federator Project Needs-Environment (ANDRA/CEA/EDF/IRSN). We are really grateful to Electricité De France (EDF) for the access of the lake Saint-Clément. The two anonymous reviewers are greatly welcomed for their constructive comments.

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