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Natural geochemical concentrations and fluxes of Cu, Th and U in Finland

¹ Geological Survey of Finland, PO Box 96, FIN-02151, Espoo, Finland (e-mail: Timo.Tarvainen@gtk.fi)
² Radiation and Nuclear Safety Authority (STUK), PO Box 14, FIN-00881 Helsinki, Finland

Concentrations of U, Th and Cu were studied as a part of the coordinated IAEA (International Atomic Energy Agency) research project on ‘The use of selected safety indicators (concentrations; fluxes) in the assessment of radioactive waste disposal’. Uranium and Th served as analogues of radionuclides in the waste matrix (spent fuel) and Cu is a construction material in the Finnish disposal approach in granitic rock. Release and migration of these components through the geosphere to the biosphere is thought to be caused and influenced by corrosion, oxidation and dissolution and subsequent transport and retardation processes in groundwater in hydraulically conducting rock fractures.

The weathering rates for Cu, U and Th were calculated on the basis of the weathering rate of base cations and the concentrations of these elements in the parent soils. The mean weathering rate for Cu in till soils is 0.21 mg m^–2 a^–1. The mean values of estimated weathering rates for Th and U are 0.17 mg m^–2 a^–1 and 0.054 mg m^–2 a^–1, respectively.

Analytical data and flux measurements from groundwater monitoring of selected springs are used to calculate the quantity of dissolved elements discharging from the aquifer. Typical fluxes for five small springs were 0.023–0.46 kg Cu km^–2 a^–1 and 0.003–0.43 kg U km^–2 a^–1. The estimated fluxes of Cu and U in headwater streams were calculated on the basis of concentration data and the rate of stream flow. The estimated fluxes in 30 streams are 0.44–1418 g/month for Cu and 0.08–285 g/month for U.

From the point of view of the use of natural elemental concentrations and fluxes as natural safety indicators in the evaluation of nuclear waste repository, the basis of this complementary or alternative concept has been corroborated by the results of this work. Elemental concentration data can be better understood in relation to the underlying geology and the prevailing geochemistry and by correlation to observed elemental fluxes. The comparison of the spatial distribution of elements in different matrices gives a qualitative estimation of the dominating fluxes.

Widespread correlation between different compartments indicates mobile element fluxes from the geosphere to the biosphere. Also fixation in sinks, in particular, for U was observed. The geochemical cycles are dominated by near-surface weathering processes. Fluxes are also controlled by small-scale, local groundwater flow patterns; contributions from greater depths were found to be negligible.

KEYWORDS: geochemical surveys, till, stream water, groundwater, weathering, Finland