1 Department of Earth and Environmental Science, College of Liberal Arts and Sciences, University of Iowa, 115 Trowbridge Hall, Iowa City, IA 52242 USA.
2 Andrews Institute of Mathematics and Science Education, Texas Christian University, TCU Box 297920, Fort Worth, TX 76129 USA.
3 Office of Research and Development, U. S. Environmental Protection Agency, 2733 South Crystal Dr. Arlington, VA 22202 USA.
4 St. Croix Watershed Research Station Science Museum of Minnesota 16910-152nd St. N Marine on St. Croix, MN 55047 USA.