3.7 Summary

Tritium is part of the water molecule and is a widely used tracer in groundwater studies. While [latex]_{}^{3}H[/latex] occurs naturally, concentrations in precipitation rose dramatically beginning in the 1950s due to nuclear weapons testing. Historically, [latex]_{}^{3}H[/latex] has been used to distinguish pre‑ from post‑bomb water.

As a groundwater dating method, [latex]_{}^{3}H[/latex] is both a radioactive and transient tracer. Absolute dating is done by extending a decay curve from the measured concentration back in time to its intersection with the curve representing the measured concentration of [latex]_{}^{3}H[/latex] in precipitation through time. However, in the northern hemisphere from about 1980 to 2000, the precipitation and decay curves are approximately parallel, leading to large ranges of possible ages. From about 2000 to present, the precipitation curve has been approximately flat—constant average concentration—making absolute dating possible for recharge years beyond 2000. For waters that fell as precipitation after 2000, the primary uncertainty in absolute dating with [latex]_{}^{3}H[/latex] is the spatial and temporal variability in the input concentration. Age uncertainties on the order of [latex]pm 5[/latex]years are typical but depend on factors such as the magnitude of annual variations.