1. Environmental NMR Center and Department of Chemistry, University of Toronto, 1265 Military Trail, Toronto, ON, M1C 1A4, Institute of Groundwater Ecology, Helmholtz Zentrum München - National Research Center for Environmental Health, Neuherberg, Germany, and Stable Isotope Laboratory, Department of Geology, University of Toronto, 22 Russell Street, Toronto, ON, M5S 3B1