1. Department of Civil and Environmental Engineering, Stanford University, Stanford, California 94305-4020, Environmental Sciences Division, Oak Ridge National Laboratory, P.O. Box 2008, Oak Ridge, Tennessee 37831, Ecovation Inc., Victor, New York 14564, and Swiss Federal Institute of Aquatic Science and Technology (EAWAG), P.O. Box 611, Ueberlandstrasse 133, CH-8600 Duebendorf, Switzerland