U.S. Department of the Interior U.S. Geological Survey Acid-rock drainage at Skytop, Centre County, Pennsylvania, 2004 by Jane M. Hammarstrom 1 , Keith Brady 2 , and Charles A. Cravotta III 3 Open-File Report 2005-1148 Any use of trade, firm, or product names is for descriptive purposes only and does not imply endorsement by the U.S. Government 1 U.S. Geological Survey, 954 National Center, Reston, VA 20192 jhammars@usgs.gov 2 PA Department of Environmental Protection, PO Box 8461, Harrisburg, PA 17105 3 U.S. Geological Survey, 215 Limekiln Road, New Cumberland, PA 17070