1. U.S. Geological Survey, Water Resources Division, 345 Middlefield Road, Mail Stop 420, Menlo Park, CA 94025
2. U.S. Geological Survey, 432 National Center, Reston, VA 22092
3. Department of Chemistry and Geochemistry, Colorado School of Mines, Golden, CO 80401
4. U.S. Geological Survey, Denver Federal Center, Mail Stop 418, Lakewood, CO 80225