1. Environmental Isotopes Group, Department of Water Resources and Drinking Water, Swiss Federal Institute of Environmental Science and Technology (EAWAG), CH-8600 Dübendorf, Switzerland, Institute of Hydromechanics and Water Resources Management, ETH Hönggerberg, CH-8093 Zürich, Switzerland, and Isotope Geology, Department of Earth Sciences, Swiss Federal Institute of Technology (ETH), CH-8092 Zürich, Switzerland