Radiochimica Acta 52/53, 367-372 (1991) © R. Oldenbourg Verlag, München 1991 - 0033-8230/91 $ 3.00+0.00 The Distribution of Natural Radionuclides on Fracture Surfaces in Palmottu Analogue Study Site in SW Finland By J. Suksi', T. Ruskeeniemi 2 , A. Lindberg 3 and T. Jaakkola 1 1 Department of Radiochemistry, University of Helsinki, SF-00170 Helsinki, Finland 2 Helsinki University of Technology, Laboratory of Engineering Geology and Geophysics, SF-02150 Espoo, Finland 3 Geological Survey of Finland, SF-02150 Espoo, Finland (Received November 6,1989; revised February 26,1990) Natural analogue / Radionuclide migration / xvutsrqonmlihgedcbaPF Ό-series disequilibrium / Fracture coating / Phase selective extraction Summary The distribution of U and Th on fracture surfaces near an uranium ore deposit was studied by the selective dissolution technique. Uranium series disequilibrium measurements and the analysis of Ca, Mg, Fe and Μη were used to interpret radio- geochemical phenomena and mechanisms which have been re- sponsible for the observed radionuclide distribution in the study site. The principal minerals identified in fracture coatings were in varying proportions kaolinite, calcite, quartz and illite. The bulk of the thorium and uranium was found in the carbonate phase. The isotopic distribution indicates uranium sorption in different groundwater conditions and recent rock- groundwater interaction. The activity ratio of uranium and tho- rium isotopes in calcite were also used to estimate the depo- sitional age of the calcite. The average ages for the two calcites were 80000 years and 300000 years or older. 1. Introduction The purpose of this work is to study the long term effects of the rock-groundwater interaction on radio- nuclide migration by examining the radionuclide dis- tribution on fracture coatings. Uranium series disequi- librium measurements and phase selective extractions have been used to identify mechanisms and phases responsible for radionuclide retardation. The work forms part of a Finnish natural analogue study project started in 1987 [1], The aim of the project is to study the mobility and retention of natural radionuclides near the uranium ore deposit as an analogy to the far field migration of nuclear waste nuclides. The study site of Palmottu is situated in SW Finland. The bedrock of the study site consists of mica gneisses, quartz-feldspar gneisses and pyroxene- bearing gneisses with abundant granitic veins (a few centimeters wide) and granitic interlayers of lensoidal bodies (from a few meters to hundreds of meters in width). The preliminary groundwater investigations indicate oxidizing conditions in the upper parts of the drill holes and reducing conditions below the depth of 145 m. Fresh hydrogen carbonate groundwater (pH 7) occurs in the upper part, and slightly saline chloride waters (pH 9), with high sulphate concentrations dominates deeper down [2]. Fractures to this study were searched in granitic interlayers and mica gneisses apart from the uranium ore. Samples from the chosen drill cores were picked up by a sensitive portable contamination meter in- stalled to measure alpha particles. The fracture sur- faces showing highest radioactivity were taken to analyses because they were believed to represent the flow paths for dissolved and particulate bound radio- nuclides. High radionuclide concentrations also offer an opportunity to observe and study marginal effects and phenomena. Around the sample depths, the groundwater conditions were found to be oxidizing [2]· 2. Materials and methods The fracture surface material was removed mechan- ically and ground gently except in the case of sample 124 where the brown thin coating was removed by dissolution. A small amount of mineral fragments de- rived from the host rock also accompanied the coat- ings. Principal minerals identified by XRD were in varying proportions kaolinite, illite, calcite and quartz. The detailed description of the samples is in Table 1. Sample amount varied between 26 mg and 121 mg. 2.1. Phase selective extractions During the last few years several procedures for ex- tracting radionuclides from different mineral phases have been published [3, 4, 5], The technique is well known in geochemical exploration when analyzing ore elements or associated pathfinding elements and is also extensively reviewed by Chao [6]. The importance of the technique, especially in studying the bioavail- ability of toxic and radioactive elements, is reflected by the fact that an international workshop dealing with topic was held in 1985 at Oxford [7], In this work the selective chemical separation scheme was chosen on the grounds of the composition of fracture coatings. It was also thought that a minor fraction of exchangeable ions could be released in Brought to you by | New York University Bobst Library Technical Services Authenticated Download Date | 9/15/15 6:59 PM