1. Branch of Regional Research − Central Region, U.S. Geological Survey, MS 408, P.O. Box 25046, Denver, Colorado 80225, National Water Quality Laboratory, U.S. Geological Survey, MS 407, P.O. Box 25046, Denver, Colorado 80225, Phillips Research Center, 113 Geoscience Building, Bartlesville, Oklahoma 74004, College of Agriculture and Natural Resources, University of Maryland, P.O. Box 169, Queenstown, Maryland 21658