1. Norwegian Institute for Water Research (NIVA), Økernveien 94, 0579 Oslo, Norway
2. Centre for Environmental Radioactivity (CERAD), Norwegian University of Life Sciences (NMBU), Post box 5003, N-1432 Ås, Norway
3. Faculty of Environmental Sciences and Natural Resource Management (MINA), Norwegian University of Life Sciences (NMBU), Post box 5003, N-1432 Ås, Norway