Aquifer Characteristics (U-Series), Figure 1

The isotopes of the ²³⁸U, ²³⁵U, and ²³²Th decay series. The isotopes in bold are those most commonly measured in groundwater studies. The dashed arrows indicate where isotopes have been left out that have very short half-lives and so are measured only in rare cases. Radioactive half-lives are given below each isotope in units of billions of years (Ga), thousands of years (ka), years (a), or days (d).