Fractionation of natural radionuclides in soils from the vicinity of a former uranium mine Z ˇ irovski vrh, Slovenia Marko S ˇ trok, Borut Smodis ˇ * Jo zef Stefan Institute, Jamova 39, SI-1000 Ljubljana, Slovenia article info Article history: Received 24 March 2009 Received in revised form 27 July 2009 Accepted 4 August 2009 Available online 16 September 2009 Keywords: Sequential extraction Natural radionuclides Alpha spectrometry Radiochemical disequilibrium abstract As a result of former uranium mining and milling activities at Z ˇ irovski vrh, Slovenia, 0.6 million tons of uranium mill tailings (UMT) were deposited onto a nearby waste pile Bors ˇt. Resulting enhanced levels of natural radionuclides in UMT could pose threat for the surrounding environment. Therefore, sequential extraction protocol was performed to assess mobility and bioavailability of 238 U, 234 U, 230 Th and 226 Ra in soils from the waste pile and its surrounding. The radionuclides associated with exchangeable, organic, carbonate, Fe/Mn oxides and residual fraction, respectively, were determined. Results showed that the highest activity concentrations for the studied radionuclides were on the bottom of the waste pile. In non-contaminated locations, about 80% of all radionuclides were in the residual fraction. Considering activity concentrations in the UMT, 238 U and 234 U are the most mobile. Mobility of 226 Ra is suppressed by high sulphate concentrations and is similar to mobility of 230 Th. Ó 2009 Elsevier Ltd. All rights reserved. 1. Introduction Industrial excavations of the uranium ore in the area of Z ˇ irovski vrh, Slovenia were started in the year 1982. Yellowcake production was started two years later and lasted up to 1990, when the mining and milling activities were stopped. During this time, about 0.6 million tons of uranium ore and more than 2.5 million tons of other waste rocks and pore uranium ore were excavated. After the uranium ore was crushed and milled in a mill, it was leached with H 2 SO 4 in order to obtain uranium from the ore. Remains after leaching are called uranium mill tailings (UMT). UMT is gray col- oured waste product from leaching and contains enhanced levels of all decay products from the 238 U decay chain. Activity concentra- tions in the UMT at Bors ˇt are: (995  80) Bq/kg of 238 U, (3930  580) Bq/kg of 230 Th and (8630  340) Bq/kg of 226 Ra (Kri zman et al., 1995). UMT was deposited onto the Bors ˇt waste pile, which lies close to the uranium mine. The amount of UMT is about 0.6 million tons. In the time of sampling, the Bors ˇt waste pile was covered with 30–40 cm thick soil cover and overgrown with grass and pine trees. The waste pile is situated in the subalpine region with relatively high rainfall and within a relatively densely populated area. Therefore, it is very important to assess mobility and bioavailability of natural radionuclides present in the tailings, both from radiation protection and long-term stability of waste pile viewpoints. Sequential extraction is one of the powerful tools for assessing mobility and bioavailability of contaminants in the environment. Since their introduction in the late 1970s, sequential extraction procedures have experienced a rapid increase in use for a large number of potentially toxic elements in a wide range of sample types (Bacon and Davidson, 2008). Nowadays two procedures have attained almost a rank of standard, one is Tessier’s method (Tessier et al., 1979) and the other is so called BCR method (Quevauviller et al., 1993). Use of sequential extraction procedures for natural radionuclide investigations is rather scarce (Bunzl et al., 1995; Blanco et al., 2005) and usually implies Tessier’s method or Schultz modification of Tessier method (Schultz et al., 1998). Blanco et al. (2004) investigates both Tessier’s method and Schultz modification of Tessier’s method and concluded that the two procedures yield different results and that Schultz modifications better suit the objectives of this type of sequential protocol. Therefore, Schultz modification of Tessier’s method was used in our study. 2. Materials and methods 2.1. Sampling and sample preparation About 5 kg of each soil sample from six sites at the Bors ˇt waste pile, presented in Fig. 1, were collected at the depth 10–15cm. The sites 1 and 2 lie in two swampy areas through which seepage water from the Bors ˇt waste pile with elevated radio- nuclides concentrations is flowing. It was expected that in these sites the radionu- clides could be retained and consequently their activity concentrations elevated. The * Corresponding author. Tel.: þ38615885353; fax: þ38615885346. E-mail addresses: marko.strok@ijs.si (M. S ˇ trok), borut.smodis@ijs.si (B. Smodis ˇ). Contents lists available at ScienceDirect Journal of Environmental Radioactivity journal homepage: www.elsevier.com/locate/jenvrad 0265-931X/$ – see front matter Ó 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.jenvrad.2009.08.006 Journal of Environmental Radioactivity 101 (2010) 22–28