1. Institute of Mineralogy, Department of Analytical Environmental Chemistry, J.W. Goethe-University, Georg-Voigt-Strasse 16, D-60054 Frankfurt am Main, Institute of Spectrochemistry and Applied Spectroscopy, Bunsen-Kirchhoff-Strasse 11, D-44139 Dortmund, Umweltbundesamt, Paul-Ehrlich-Strasse 29, D-63225 Langen, and Hessisches Landesamt für Umwelt und Geologie, Rheingaustrasse 186, D-65203 Wiesbaden, Germany