1. GSF-National Research Center for Environment and Health, Institute of Groundwater Ecology, Ingolstädter Landstraße 1, D-85764 Neuherberg, Germany, Department of Geoscience, University of Calgary, Calgary, Alberta, Canada T2N 1N4, and Forschungszentrum Karlsruhe, Institut für Nukleare Entsorgung (INE), P.O. Box 3640, D-76021 Karlsruhe, Germany