1. Instituto de Recursos Naturales y Agrobiología (IRNA), C.S.I.C., Apartado 1052, E-41080 Seville, Spain, Swiss Federal Institute for Environmental Science and Technology (EAWAG), Überlandstrasse 133, CH-8600 Dübendorf, Switzerland, and Vlaamse Instelling voor Technologisch Onderzoek (VITO), Boeretang 200, B-2400 Mol, Belgium