1. Institute of Environment & Resources, Technical University of Denmark, DK-2800 Kongens Lyngby, Denmark, Istituto di Chimica Agraria ed Ambientale, Università Cattolica del Sacro Cuore, I-29100 Piacenza, Italy, and National Environmental Research Institute, University of Aarhus, PO Box 358, DK-4000 Roskilde, Denmark