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 (H/He) 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 anthropogenic sources) those sources of tritium usually play only a secondary role in the analysis of groundwater.
* [http://water.usgs.gov/lab/3h3he/background/ USGS Tritium/Helium-3 Dating]
* [http://wwwrcamnl.wr.usgs.gov/isoig/period/he_iig.html Hydrologic Isotope Tracers - Helium]