Vertical migration of radionuclides in undisturbed grassland soils Gerald Kirchner a , Friederike Strebl b, * , Peter Bossew c , Sabine Ehlken d , Martin H. Gerzabek e a BfS Federal Office for Radiation Protection, D-38201 Salzgitter, Germany b Austrian Research Centers GmbH (ARC), Department of Radiation Safety and Applications, A-2444 Seibersdorf, Austria c Institute for Environment and Sustainability, DG Joint Research Centre, European Commission , I-21020 Ispra, Italy d Klinikum Bremen-Mitte, D-28177 Bremen, Germany e University of Natural Resources and Applied Life Sciences Vienna, Peter-Jordan-Strasse 82b, A-1190 Vienna, Austria article info Article history: Received 26 April 2007 Received in revised form 27 August 2008 Accepted 7 October 2008 Available online 25 November 2008 Keywords: Caesium CDE models Convection–dispersion equation Model parameters Radionuclides Vertical migration abstract Literature data on numerical values obtained for the parameters of the two most popular models for simulating the migration of radionuclides in undisturbed soils have been compiled and evaluated statistically. Due to restrictions on the applicability of compartmental models, the convection–dispersion equation and its parameter values should be preferred. For radiocaesium, recommended values are derived for its effective convection velocity and dispersion coefficient. Data deficiencies still exist for radionuclides other than caesium and for soils of non-temperate environments. Ó 2008 Elsevier Ltd. All rights reserved. 1. Introduction For radionuclides present in terrestrial ecosystems, information about their migration in soils is crucial, since this process controls their long-term behaviour in the environment, their uptake by flora and fauna including human food chains, but also their potential as a groundwater contaminant. The basic processes controlling mobility of radionuclides (and other trace elements) in soil include convective transport by flowing water, dispersion caused by spatial variations of convection velocities, diffusive movement within the fluid, and physico-chemical interaction with the soil matrix. In addition to these abiotic processes, soil fauna may contribute to the transport of radionuclides in soils (Mu ¨ ller-Lemans and van Dorp, 1996), but their action under general conditions results in a dispersion-like translocation (Boudreau, 1986). After the Chernobyl-derived contamination it soon became evident that radiocaesium behaviour in semi-natural environments differs significantly from that in agricultural areas. Organic soil horizons, high microbial/fungal biomass in soils (Johanson and Nikolova, 1996; Stemmer et al., 2005) and dense and superficial rooting systems are important for Cs retention in semi-natural soils under highland or alpine meadows (and forests). Since the early 1960s scientific interest has focused on pre- dicting the environmental fate of isotopes with long half-lives, most prominently 90 Sr, 137 Cs and 238–240 Pu. These radioisotopes more or less strongly sorb to the soil matrix, showing K d -values usually well above 100 cm 3 g 1 . Thus transport velocities in the unsaturated zone are about 1 cm y 1 or lower, indicating that a major fraction of the originally deposited activity may remain within the rooting zone for decades. Because of this slow migration velocity, models that simulate radionuclide movement in soils usually do not take into account soil moisture changes in the unsaturated zone, but (often implicitly) assume a constant mean water content. A second simplification is the assumption of spatial uniformity of deposition rates, which may be justified for the weapons testing fallout and, at least on a local scale, also for the Chernobyl fallout. With these simplifications, two approaches have become most popular for modelling the migration of radionuclides in soils – a serial compartmental approach and the one-dimen- sional convection–dispersion equation. Focusing on these models, the parameters needed for radionuclide transport simulation in undisturbed grassland soils are reviewed in the present paper. Discussion of the observations on the fixation of radionuclides in arable soils is not the scope of this paper and can be found elsewhere (Coughtrey et al., 1983; Gerzabek et al., 2003). * Corresponding author. Tel.: þ43 (0)50550 3265; fax: þ43 (0)50550 2502. E-mail address: friederike.strebl@arcs.ac.at (F. Strebl). Contents lists available at ScienceDirect Journal of Environmental Radioactivity journal homepage: www.elsevier.com/locate/jenvrad 0265-931X/$ – see front matter Ó 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.jenvrad.2008.10.010 Journal of Environmental Radioactivity 100 (2009) 716–720