1. Eawag, Swiss Federal Institute of Aquatic Science and Technology, 8600 Dübendorf, Switzerland, Brunel University, Uxbridge, Middlesex UB8 3PH, U.K., Empa, Swiss Federal Laboratories for Materials Testing and Research, 8600 Dübendorf, Switzerland, and Institute for Chemistry and Dynamics of the Geosphere: Institute 3, Phytosphere, Research Centre Juelich, D-52425 Juelich, Germany