Which isotope is commonly used for dating groundwater?

• A. Tritium
• B. Uranium-238
• C. Potassium-40
• D. Rubidium-87

Answer: (A)

Tritium is used for dating groundwater because it provides a practical time scale for recent recharge. Its radioactive half-life is about 12.3 years, so its amount in water declines in a predictable way as groundwater ages, making it useful for ages from a few years up to several decades. The big tap of tritium in the hydrologic cycle came from atmospheric nuclear weapons testing in the 1950s–1960s, which created a known, time-marked pulse in precipitation that eventually enters and mixes with groundwater. By measuring tritium (and the helium-3 produced as it decays), scientists can estimate when the water was last recharged.

The other isotopes listed are tied to much longer timescales and mineral ages. Uranium-238, Potassium-40, and Rubidium-87 are typically used to date rocks and minerals over millions to billions of years, and their systems don’t reset or respond in the way groundwater does, so they aren’t the standard tools for dating groundwater in the modern to decadal range.