3. Tritium Dating Method
Tritium ([latex]_{}^{3}H[/latex], T) is a radioactive isotope of hydrogen ([latex]H[/latex]) that is produced naturally in both the atmosphere and subsurface and is also produced by various nuclear activities including nuclear reactors, weapons testing, and nuclear fuel reprocessing facilities (Michel et al., 2018). In the atmosphere, [latex]_{}^{3}H[/latex] readily oxidizes to form tritiated water—typically [latex]HTO[/latex]—and has a half‑life of 12.32 years (Lucas & Unterweger, 2000).
Because it is a part of the water molecule, [latex]_{}^{3}H[/latex] is nearly an ideal tracer for water. The [latex]_{}^{3}H[/latex] content of water is typically expressed in tritium units (TU) which is 1 atom of [latex]_{}^{3}H[/latex] in 1018 atoms of hydrogen ([latex]_{}^{1}H[/latex], [latex]_{}^{2}H[/latex], and [latex]_{}^{3}H[/latex]). One TU has an approximate activity of 0.118 Becquerels per kilogram (Bq kg–1; Stonestrom et al., 2013). A Becquerel is 1 disintegration per second, which is equal to 3.19 picocuries per kilogram (pCi/kg). The use of [latex]_{}^{3}H[/latex] as a tracer in groundwater was summarized by Solomon and Cook (2000).