past circulation changes. Anthropogenic isotopes Tritium/helium-3 Tritium was released to the atmosphere during atmospheric testing of nuclear bombs. Radioactive decay of tritium produces the noble gas helium-3. Comparing the ratio of tritium to helium-3 (3H/3He) allows estimation of the age of recent ground waters. A small amount of Tritium is also produced naturally by cosmic ray spallation and spontaneous ternary fission in natural uranium and thorium, but due to the relatively short half-life of Tritium and the relatively small quantities (compared to those from humandmade sources) those sources of Tritium usually play only a secondary role in the analysis of groundwater. USGS Tritium/Helium-3 Dating Hydrologic Isotope Tracers - Helium See also Cosmogenic isotopes Environmental isotopes Geochemistry Isotopic signature Radiometric dating Isotope-ratio mass spectrometry Sulfur isotope biogeochemistry Urey-Bigeleisen-Mayer equation Notes References General Allègre C.J., 2008. Isotope Geology (Cambridge University Press). Dickin A.P., 2005. Radiogenic Isotope Geology (Cambridge University Press). Faure G., Mensing T.M. (2004), Isotopes: Principles and Applications (John Wiley & Sons). Hoefs J., 2004. Stable Isotope Geochemistry (Springer Verlag). Sharp Z., 2006. Principles of Stable Isotope Geochemistry (Prentice Hall). Stable isotopes Environmental Isotopes (University of Ottawa) Fundamentals of Isotope Geochemistry (C. Kendall & E.A. Caldwell, chap.2 in Isotope Tracers in Catchment Hydrology [edited by C. Kendall & J.J. McDonnell], 1998) Stable Isotopes and Mineral Resource Investigations in the United States (USGS) 3He/4He Re–Os External links National Isotope Development Center Reference information on isotopes, and coordination and management of isotope production, availability, and distribution Isotope Development & Production for Research and Applications (IDPRA) U.S. Department of Energy program for isotope production and production research and development Geochemistry Geophysics Geochronological dating methods