1. Department of Geosciences and Department of Chemical Engineering, Virginia Tech, Blacksburg, Virginia 24061, Department of Geosciences, University of Arizona, Tucson, Arizona 85721, Department of Mathematics, University of Idaho, Moscow, Idaho 83844-1103, and Chemical and Materials Science Division, and the W.R. Wiley Environmental Molecular Sciences Laboratory, Pacific Northwest National Laboratory, Richland, Washington 99352