Kari Rasilainen

U-series disequilibria in a groundwater flow route as an indicator of uranium migration processes

U-series data relating to groundwater, fracture coatings and the adjoining rock matrix in a groundwater flow system at the Palmottu natural analogue site was examined. The aim was to obtain an experimental reference for migration modelling in a transport section defined within the flow system. The U-series reference obtained turned out to be a very useful tool for fine tuning the flow route and for migration mechanism considerations. The U-series data are well in line with other interpretations of the migration system.

Characterization of the Altered Zone Around a Fracture in Palmottu Natural Analogue

A drill core sample penetrating at a natural fracture has been analysed by helium gas techniques and by α-autoradiography. Porosity and diffusivity profiles away from the fracture have been determined and compared with structural profiles. Model calculations are used to determine the effects of sample size on the measured porosities and diffusivities.

Release of uranium from rock matrix--a record of glacial meltwater intrusions?

Uranium release observed in a rock matrix around water-carrying fractures was studied using U-series disequilibrium (USD) modelling and mass balance calculations. Several release scenarios were tested, with specific attention to the glacial aspects. The release appears to have occurred in two or three violent episodes during the last 300 ky. A release after the last glaciation can be excluded on mass flow grounds. Continuous release for more than 300 ky can be excluded on radioactive disequilibrium grounds. Repeated inflows of oxic glacial meltwater seem to have triggered the release episodes.

A multisystem modeling approach for uranium-series dating

The sensitivity of the dating of long-term uranium attachment to bedrock fractures was studied using radioactive disequilibria in the natural 238 U decay chain. For this purpose, fracture coating samples were taken from the uranium deposit at Palmottu, Finland. A general simulation model was derived for the evolution of the 234 U/ 238 U and 230 Th/ 234 U disequilibria. Instantaneous, continuous, and multistage uranium accumulation modes were tested to see their effect on the model age. All accumulations produced different, but internally consistent, model ages, except the multistage scenario that yielded a nonunique dating. The simulation model and scenario technique provide a good modeling approach, and the real challenge in uranium-series dating appears to be the quantification of the accumulation rates.

Postglacial Matrix Diffusion in a Boulder Sample

A boulder sample was studied for its unusual U content. Analyses of U-series nuclides within the rock matrix perpendicular to an assumed fracture face show abrupt pulse-like concentration distributions with very low concentrations of U daughters. Both Th-230/U-234 and Pa-231/U-235 activity ratios are low, indicating recent U accumulation into the rock. Matrix diffusion is tested as a possible cause for the experimental observations. The authors assume that the diffusion process was triggered and controlled by rock expansion, strong mixing of different water types and rapid land uplift at the end phase of the last glaciation.

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.

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.

Sorption Aspects for in Situ Matrix Diffusion Modelling at Palmottu Natural Analogue Site, Sw Finland

Concentration profiles in rock matrix around water-carrying fissures were measured at Palmottu U deposit. The profiles were interpreted by the classical matrix diffusion concept. Site-specific sorption studies were performed for U using standard batch experiments and surface complexation modeling; the response of sorption isotherms was also tested. Site-specific matrix properties as well as initial and boundary conditions were used in simulations. The results indicate that matrix diffusion alone cannot explain the observed enrichment of U and its daughters in the rock matrix.

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.