Distribution and transport of radionuclides in a boreal mire--assessing past, present and future accumulation of uranium, thorium and radium

On the detailed mapping of peat (raised bogs) using airborne radiometric data

This study concerns the applied use of the natural radioactivity in soils. The relevance of airborne radiometric (gamma ray) survey data to peat mapping is now well established and such data have been used in a stand-alone sense and as covariates in machine learning algorithms. Here we present a method to use these data to accurately map the boundaries of peat (raised bogs). This has the potential to assist with the estimation of carbon stocks using a property-based assessment of soil. The significance of such regionally-uniform survey data lies in the subsurface information carried by the measurement which contrasts with the surficial nature of many other covariates. Soils attenuate radiometric flux by virtue of their bulk density (and associated carbon content) and water saturation level. The high attenuation levels in low density, wet peat materials give rise to a distinctive soil response. Here an entirely physics-based assessment of flux attenuation is carried out both theoretically and empirically. Radiometric data from the ongoing Tellus airborne survey of Ireland are used. The study area is characterised by an extensive assemblage of discrete raised peat bogs in a framework of largely mineral soils. Peat is detected by a property contrast with adjacent soils and so we consider all soils within the study area. The relatively low lateral resolution of the airborne data is demonstrated by modelling and we examine the behaviour of a combined spatial derivative of the data. The procedure allows the identification of the edges of the 128 peat polygons considered and indicates other additional potential areas of subsurface peat. The data appear to resolve the differences that exist across three available soil/peat databases that are used for the validation of the results obtained.

Convergence of Groundwater Discharge through the Hyporheic Zone of Streams

Significant attention has been given to hyporheic water fluxes induced by hydromorphologic processes in streambeds and the effects they have on stream ecology. However, the impact of hyporheic fluxes on regional groundwater flow discharge zones as well as the interaction of these flows are much less investigated. The groundwater-hyporheic interactive flow not only governs solute mass and heat transport in streams but also controls the retention of solute and contamination following the discharge of deep groundwater, such as naturally occurring solutes and leakage from geological waste disposal facilities. Here, we applied a physically based modeling approach combined with extensive hydrologic, geologic and geographical data to investigate the effect of hyporheic flow on groundwater discharge in the Krycklan catchment, located in a boreal landscape in Sweden. Regional groundwater modeling was conducted using COMSOL Multiphysics by considering geologic heterogeneity and infiltration constraint of the groundwater circulation intensity. Moreover, the hyporheic flow was analyzed using an exact spectral solution accounting for the fluctuating streambed topography and superimposed with the regional groundwater flow. By comparing the discharge flow fields with and without consideration of hyporheic flows, we found that the divergence of the discharge was substantially enhanced and the distribution of the travel times of groundwater was significantly shifted toward shorter times due to the presence of hyporheic flow. Particularly important is that the groundwater flow paths contract near the streambed interface due to the hyporheic flow, which leads to a phenomenon that we name "fragmentation" of coherent areas of groundwater upwelling in pinhole-shaped stream tubes.

Geochemical controls on dispersion of U and Th in Quaternary deposits, stream water, and aquatic plants in an area with a granite pluton

The weathering of U and/or Th rich granite plutons, which occurs worldwide, may serve as a potentially important, but as yet poorly defined source for U and Th in (sub-)surface environments. Here, we assessed the impact of an outcrop of such granite (5 km in diameter) and its erosional products on the distribution of U and Th in four nemo-boreal catchments. The results showed that (i) the pluton was enriched in both U and Th; and (ii) secondary U and Th phases were accumulated by peat/gyttja and in other Quaternary deposits with high contents of organic matter. Movement of the ice sheet during the latest glaciation led to dispersal of U- and Th-rich materials eroded from the pluton, resulting in a progressive increase in dissolved U and Th concentrations, as well as U concentrations in aquatic plants with increasing proximity to the pluton. The accumulation of U in the aquatic plants growing upon the pluton (100-365 mg kg^-1, dry ash weight) shows that this rock represents a long-term risk for adjacent ecosystems. Dissolved pools of U and Th were correlated with those of dissolved organic matter (DOM) and were predicted to largely occur as organic complexes. This demonstrates the importance of DOM in the transport of U and Th in the catchments. Large fractions of Ca₂UO₂(CO₃)₃⁰(aq) were modeled to occur in the stream with highest pH and alkalinity and thus, explain the strongly elevated U concentrations and fluxes in this particular stream. In future climate scenarios, boreal catchments will experience intensified runoff and warmer temperature that favor the production of hydrologically accessible DOM and alkalinity. Therefore, the results obtained from this study have implications for predicting the distribution and transport of Th and U in boreal catchments, especially those associated with U and/or Th rich granite plutons.

Chemical reactivity of natural peat towards U and Ra

Peat is a complex material with several organic constituents that contribute to its high capacity to retain metals. In the context of uranium mining, peat can accumulate high concentrations of uranium and its decay products such as radium. Hence, interaction with peat appears to be a key factor in the understanding of the geochemical mechanisms controlling the fate of these products. This study aims to determine the sorption properties of two trace elements, U(VI) and ²²⁶Ra, on natural organic matter from peat. The presented method was applied to both natural peat samples originating from a mining context, with various contents of organic matter (from 40 to 70%) and detrital loads, and wetland peat with a more than 98% composition of organic matter. In the present study, considering peat material as a sorbent, its reactivity towards metals and other contaminants can be described as that of an ion-exchanger. A relatively simple model of ion-exchange based on the sorption properties of carboxylic sites has been applied with success to describe the sorption of uranium and radium. In the general overview of the different mechanisms able to control the mobility of these radionuclides in a uranium mining context, organic matter is likely one of the main contributors to radionuclide scavenging even under oxic conditions.

Tetra- and Hexavalent Uranium Forms Bidentate-Mononuclear Complexes with Particulate Organic Matter in a Naturally Uranium-Enriched Peatland

Peatlands frequently serve as efficient biogeochemical traps for U. Mechanisms of U immobilization in these organic matter-dominated environments may encompass the precipitation of U-bearing mineral(oid)s and the complexation of U by a vast range of (in)organic surfaces. The objective of this work was to investigate the spatial distribution and molecular binding mechanisms of U in soils of an alpine minerotrophic peatland (pH 4.7-6.6, E_h = -127 to 463 mV) using microfocused X-ray fluorescence spectrometry and bulk and microfocused U L₃-edge X-ray absorption spectroscopy. The soils contained 2.3-47.4 wt % organic C, 4.1-58.6 g/kg Fe, and up to 335 mg/kg geogenic U. Uranium was found to be heterogeneously distributed at the micrometer scale and enriched as both U(IV) and U(VI) on fibrous and woody plant debris (48 ± 10% U(IV), x̅ ± σ, n = 22). Bulk U X-ray absorption near edge structure (XANES) spectroscopy revealed that in all samples U(IV) comprised 35-68% of total U (x̅ = 50%, n = 15). Shell-fit analyses of bulk U L₃-edge extended X-ray absorption fine structure (EXAFS) spectra showed that U was coordinated to 1.3 ± 0.2 C atoms at a distance of 2.91 ± 0.01 Å (x̅ ± σ), which implies the formation of bidentate-mononuclear U(IV/VI) complexes with carboxyl groups. We neither found evidence for U shells at ∼3.9 Å, indicative of mineral-associated U or multinuclear U(IV) species, nor for a substantial P/Fe coordination of U. Our data indicates that U(IV/VI) complexation by natural organic matter prevents the precipitation of U minerals as well as U complexation by Fe/Mn phases at our field site, and suggests that organically complexed U(IV) is formed via reduction of organic matter-bound U(VI).

Study of uranium(VI) and radium(II) sorption at trace level on kaolinite using a multisite ion exchange model

Uranium and the long-lived decay product radium-226 are abundantly present in mine wastes produced during uranium extraction activities. In the case of release to the surrounding environment, these radionuclides are at trace level compared to groundwater solutes, and the presence, content and properties of clay minerals in the host environment influence the extent of radionuclide sorption and, in turn, migration. Since clays are known to have the distinctive property of retaining ions, the aim of this work was to study the sorption of trace U(VI) and Ra(II) on a common phyllosilicate mineral, kaolinite, in the presence of excess K, a common groundwater cation, in order to obtain a thermodynamic database that describes the ion exchange equilibria occurring at the mineral-solution interface. Following a detailed experimental protocol using chemical and radiochemical analytical techniques, batch experiments over a wide pH range (from 2 to 11) and fixed concentration (ca. 10(-9) M), and additional adsorption isotherms at two different solution pH (6.2 and 10.4) over a concentration range (10(-10) to 10(-4) M) were carried out to measure the distribution coefficient (Kd) of U(VI) and Ra(II) sorption on kaolinite. The experimental sorption data was processed according to a general multisite sorbent/multispecies sorbate ion exchange model, which allowed deducing the charge of adsorbed species and the stoichiometry of the associated adsorption equilibria on kaolinite's surface sites. Aqueous speciation calculations predicted Ra(II) as Ra(2+) over the working pH range, and its adsorption curves and isotherms were explained using three sorption sites. Adsorption of U(VI) occurred on four sorption sites and was governed by its solution speciation, with positively charged hydroxylated (UO2(2+) and UO2(OH)(+)) and silicate (UO2(H3SiO4)(+)) species being adsorbed between pH 2 and 6, whereas its negatively charged forms (UO2(OH)3(-) and UO2(OH)4(2-)) dominated U(VI) sorption at pH > 7. Nonlinear fitting of the experimental data using the ion exchange model provided the associated equilibrium constants as corrected selectivity coefficients.

Metal transport in the boreal landscape-the role of wetlands and the affinity for organic matter

Stream water concentrations of 13 major and trace elements (Al, Ba, Ca, Cr, Cu, La, Mg, Na, Ni, Si, Sr, U, Y) were used to estimate fluxes from 15 boreal catchments. All elements displayed a significant negative correlation to the wetland coverage, but the influence of wetlands was stronger for organophilic metals; 73% of the spatial differences in the normalized element fluxes could be explained based only on the wetland coverage and the affinity for organic matter, which was quantified using thermodynamic modeling. When the analysis was restrained to the smaller streams (<10 km(2)) the explanatory power increased to 88%. The results suggest that wetlands may decrease the fluxes of metals from boreal forests to downstream recipients by up to 40% at otherwise similar runoff. We suggest that the decrease in element fluxes is caused by a combination of low weathering in peat soils and accumulation of organophilic metals in peat. The model could not explain the spatial patterns for some metals with low affinity for organic matter, some redox-sensitive metals, and some metals with exceptionally high atmospheric deposition, but the results still demonstrate that wetlands play an important role for the biogeochemical cycling of many metals in the boreal landscape.