Nuria Marcos

Discussion on the Use of Matrix Diffusion Model after a Multidisciplinary Study of a Granitic Boulder Sample

This paper aims to discuss the applicability of the classical matrix diffusion model against the integrated body of new data obtained by different methodologies on several samples of three granite boulders. The matrix diffusion model was tested against observations from the upper (most weathered in contact with air) and lower (fresh in contact with the ground) part of a boulder block. A U(VI) enrichment up to nearly 300 ppm (compared to about 10 ppm background concentration) mostly as uranophane was observed in the zone between the weathered and fresh rock. U-series disequilibrium studies indicated that most of U has been accumulated recently, about 10 000 years ago [1]. High interconnected porosity (total porosity of > 1% and up to about 5.5% in altered minerals) characterizes the weathered zone (upper part), whereas the maximum porosity values in the fresh zone (lower part) of the rock are about 0.4 – 0.6%. Stable isotope studies δ 18 O and δ 2 H confirm that the mineralogical changes observed in the weathered upper part are due to old hydrothermal events. That is, the alteration is much older than uranium accumulation. Mossbauer spectroscopy showed that the Fe(III) content of the biotites from the upper to the lower part decreases from 30% in the weathered zone to 17% in the fresh rock, thus indicating possible redox control for the observed U precipitation. Fission track studies showed that secondary U(VI) also occurs within minerals grains (especially plagioclase) in the upper part. Mathematical simulations indicate that matrix diffusion alone is not enough to reconstruct the past U accumulation. The simulated concentrations derived from U concentration in pore water multiplied by Kd are clearly too small, indicating apparent insufficiency of the Kd approach. However, even with only matrix diffusion, the simulations roughly reconstruct the observation that U levels are clearly higher in the upper part of the boulder than in the lower part.

Release of U, REE and Th From Palmottu Granite

Interpretation of trace metal mobility in geological environments is often hampered by conflicting data from alternative experimental protocols and the lack of detailed mineralogical characterization of the host medium. To illustrate this issue, the release of uranium, thorium and the rare earth elements (REE) was investigated in polished rock slab samples from the U-Th deposit at the Palmottu Natural Analogue study site (SW Finland) by means of leaching experiments. The samples were sequentially leached with artificial groundwater of moderately high carbonate content at pH8, and nitric acid solutions at pH5 and pH3. The mineralogy and composition of the U, Th and REE mineral phases was studied using SEM-WDS and EDAX methods before and after each leaching step. In parallel, leaching was carried out on crushed material of the same samples and the leachates analysed by ICP-MS. The most notable U minerals are uraninite, uranophane and two secondary U-Pb phases. Thorium occurs predominantly in monazite and at lower concentrations in uraninite. Accessory thorite is also present, which together with monazite contains most of the REE. Differential leaching of the elements was noted across all phases on the timescale of the experiments. Uraninite is partly dissolved at pH3. The main secondary uranium phase, uranophane, was stable in moderately acidic solution, but easily dissolved in the artificial groundwater and at pH3. Some release of REE was observed although the main REE-bearing phase, monazite, showed no evidence of degradation. This study provides insights in the preferential release of radionuclides in granitic bedrock. An understanding of these processes is essential when assessing the safety of a spent fuel repository. Once released from the primary waste form U is expected to precipitate as secondary phases within micro fractures, as observed at Palmottu and numerous other deposits.

U isotopic fractionation — a process characterising groundwater systems

The activity of 234U relative to 238U (AR) in groundwaters is controlled by isotope fractionation in water-rock interface. The fractionation is controlled by redox-conditions and rock-groundwater contact time. The measured ARs form thus an important source of information and offer an effective tool to characterise groundwater systems along with other hydrochemical data. The purpose of this paper is to elucidate the link between ARs and groundwater conditions. The paper considers the formation of AR in groundwaters and examines measured data from several study sites in the Fennoscandian Shield.

Natural U occurrences as a palaeo-hydrogeological indicator — observations from the Palmottu natural analogue site, Finland

Use of natural U in studying recharge of oxygenated waters in crystalline bedrock is discussed. Uranium is a redox-sensitive element and therefore easily mobilised when conditions change from reducing to oxidising. It has two independent decay chains and occurs easily accessible to groundwater in the bedrock, thus forming a sensitive network of “probes”. Glacial melt waters rich in dissolved oxygen may penetrate the bedrock, producing signals in the U network along and around the flow routes. The signals record U behaviour as U-series disequilibria that are detectable long after the forming of the signal. Relative roles of modern groundwater flow and glacial melt waters as forming these signals are considered.