Which two isotopes are often used for age dating groundwater?

The use of tritium and carbon for age dating groundwater is based on their unique properties and behaviors in the hydrological cycle. Tritium, a radioactive isotope of hydrogen, is particularly useful for dating groundwater that is less than 50 years old. It is produced in the atmosphere by cosmic rays and incorporates into precipitation, providing a way to trace the age of relatively modern groundwater.

Carbon, specifically carbon-14, is another critical isotope used in dating, particularly for organic materials. While primarily associated with dating archaeological samples, carbon-14 can also be used to date groundwater when it is in equilibrium with the organic matter in sediment or when it has interacted with soil processes.

Together, tritium and carbon isotopes complement each other in determining the relatively short-term age of groundwater, distinguishing it from older groundwater that does not contain measurable amounts of tritium. This combination has made them a staple in hydrology and environmental studies for understanding water movement and residence times in aquifers.

In contrast, other isotopes listed, such as uranium and lead, are better suited for dating much older geological materials and rocks, while oxygen and hydrogen isotopes are generally used to understand water sources and movement rather than directly dating the water itself.