1. Center for the Study of Metals in the Environment, Department of Civil and Environmental Engineering, University of Delaware, Newark, Delaware 19716, Agriculture and Environment Division, Rothamsted Research, Harpenden, Hertfordshire, AL5 2JQ, UK, Laboratory of Environmental Toxicology and Aquatic Ecology, Ghent University, 9000 Ghent, Belgium, and Division of Soil and Water Management, K. U. Leuven, 3001 Heverlee, Belgium