1. US Geological Survey, Denver Federal Center, Box 25046, MS 964, Denver, CO 80225, USA.
2. Corresponding author.
3. EDCON-PRJ, Inc., 171 South Van Gordon Street, Suite E, Lakewood, CO 80228, USA.
4. US Geological Survey, 345 Middlefield Road, MS 989, Menlo Park, CA 94025, USA.