1. Surface and Aqueous Geochemistry Group, Department of Geological and Environmental Sciences, Stanford University, Stanford, California 94305-2115, Frontier Geosciences, 414 Pontius North, Seattle, Washington 98109, U.S. Geological Survey, 345 Middlefield Road, MS 901, Menlo Park, California 94025, and Stanford Synchrotron Radiation Laboratory, SLAC, 2575 Sand Hill Road, MS 99, Menlo Park, California 94025