1. Empa, Swiss Federal Laboratories for Materials Testing and Research, Technology and Society Laboratory, Ueberlandstrasse 129, CH-8600 Duebendorf, Switzerland, ETH Zurich, Institute of Environmental Engineering, CH-8093 Zurich, Switzerland, and Department of Environmental Science, Institute for Wetland and Water Research, Faculty of Science, Radboud University Nijmegen, P.O. Box 9010, NL-6500 GL Nijmegen, The Netherlands