1. U.S. Environmental Protection Agency, National Enforcement Investigations Center, Building 25, Denver Federal Center, P.O. Box 25227, Denver, Colorado 80225, U.S. Geological Survey Central Region, Denver Federal Center, MS 973, Denver, Colorado 80225, and.S. Environmental Protection Agency, Office of Research and Development, National Homeland Security Research Center, E343-06, 109 T.W. Alexander Drive, Research Triangle Park, North Carolina 27711B