Juhani Suksi

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.

Deposition of gamma emitters from Chernobyl accident and their transfer in lichen-soil columns.

Lichen-soil column samples were taken from several locations in the Southern Finland between 1986 and 2006. Columns were divided into three parts, upper lichen, lower lichen and underlying soil, and their gamma emitting radionuclides, 134Cs, 137Cs, 103Ru, 95Zr, 106Ru, 110mAg, 125Sb and 144Ce, were measured with gamma spectrometry. Deposition values were calculated as Bq/m2 for each sampling site. Distribution of various radionuclides in the three compartments as a function of time was determined. Both effective and ecological half-lives of all radionuclides were calculated for upper lichen, whole lichen and whole lichen-soil column. A linear relation was derived between the physical half-lives and effective half-lives for whole lichen and for whole lichen-soil column. Reindeer meat activity concentrations of various radionuclides and ensuing radiation doses to reindeer-herding people were also estimated for a hypothetical case where a similar high radioactive pollution, as was taken place in the Southern Finland, would have occurred in the reindeer-herding areas in the Finnish Lapland.

Palaeolimnological and limnogeochemical features in the sedimentary record of the polluted Lake Lippajärvi in Southern Finland

Abstract A sediment core from Lake Lippajarvi, sampled by freezing it in situ , was investigated. The sedimentary sequence was dated using the 137 Cs method and the 210 Pb content of the core was also determined. It seems that, during the recent history of the lake, increasing eutrophication is the main factor controlling fluctuations in lake ecosystems. This increase is clearly reflected in the changes in the diatom stratigraphy of the core investigated. Sequential extraction analysis was performed for speciation of elements in the sediment. Ammonium acetate and hydroxylammonium chloride plus acetic acid were used as extractants; acid digestion and extraction by NaClO and NaDDTC/MIBK were also performed. An increase in sedimentation rate is apparent for the upper part of the lithostratigraphy of Lake Lippajarvi. The increased nutrient load and heavy metal input have led to cultural eutrophication and pollution effects in the lake, and black bands of Fe monosulphides have emerged in sediments as a sign of oxygen deficit in the hypolimnion. The eutrophication of Lake Lippajarvi is also accompanied by biogeochemical cycles and limnogeochemical processes apparently responsible for the leaching of chemical elements from sediments and their subsequent redeposition in more loosely bound forms. Fe has been redeposited mainly as Fe oxyhydroxides and monosulphides, and Cu and Zn also seem to be associated with these precipitates. Ca, Mn and Zn are present to relatively large degrees as very loosely bound and sorbed forms. Fe, Mn, Cu and Zn are also associated with sulphides, and Fe and Cu with biogenic matter. There is a clear difference between man-made airborne forms of Pb and natural mineral-bound forms in the sediment column of the lake. In the polluted part of the investigated lithostratigraphy, Pb is mainly present in loosely bound reducible forms. Al and Si also seem to show leaching and redeposition in the sediments of Lake Lippajarvi.

Selective extractions in uranium migration studies—findings from a natural analogue study at Palmottu, southern Finland

The usefulness of chemical extractions in the study of uranium migration through fractured rock is considered in the light of the results of a natural analogue study conducted at Palmottu in southern Finland. Chemical extractions provide a method for producing information of the mass transfer of U and its fixation in geochemical cycling. The present detailed study provides more specific information on the fixation of U on silicate material and its sorption in rock pores. The many observations made at the Palmottu study site show the potential of the method for filling the gap in interpretations between the natural partitioning of U as a results of water-rock interaction and the distribution of artificial tracers determined by the short-term laboratory experiments.

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.

The Long-Term Stability of Becquerelite

Uranium-series disequilibria data, in conjunction with petrographic analyses, indicate that the uranyl oxide hydrate becquerelite can persist for hundreds of thousands of years, possibly longer. Becquerelite probably forms continuously as ground water compositions permit and is resistant to U leaching by ground water. On the time scale of interest for the geologic disposal of spent UO{sub 2} nuclear fuel, becquerelite is a long-lived sink for uranium in oxidizing, U and Ca-bearing ground waters. Such long-term stability also supports recent solubility experiments that indicate natural becquerelite has a lower solubility product than that determined for synthetic becquerelites.

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.

DECSERVIS: a tool for radioactive decay series visualisation

We have developed an interactive visualisation tool, decay series visualisation (DECSERVIS), for exploring the three natural radioactive decay chains. Through DECSERVIS, one can investigate the full decay scheme of any natural decay chain radionuclide to obtain the number of nuclides, their masses, activities, and activity ratios, accounting for all the daughters, starting from initial conditions freely chosen by the user. The tool has been developed particularly for user friendly and flexible operation. Chain decay in closed systems can be explored as a function of time with various graphical presentations such as solid curve and column diagrams or animation. We present several exploration examples related to geological dating. DECSERVIS will be freely available on request.

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.