1 Department of Isotope Biogeochemistry, Helmholtz Centre for Environmental Research - UFZ, Permoserstraße 15, 04318 Leipzig, Germany. Electronic address: hans.richnow@ufz.de.
2 Institute of Groundwater Ecology, Helmholtz Zentrum München - German Research Center for Environmental Health, Ingolstädter Landstr. 1, 85764 Neuherberg, Germany. Electronic address: tillmann.lueders@helmholtz-muenchen.de.