Which two isotopes are often used for age dating groundwater?

B is the correct choice because tritium and carbon isotopes, specifically carbon-14, are commonly used in dating groundwater. Tritium (a hydrogen isotope) is useful for determining the age of modern groundwater due to its relatively short half-life of about 12.3 years. This allows scientists to assess how long it takes for the water to move through the hydrological cycle and can provide insights into the age of the water currently in aquifers.

Carbon-14, on the other hand, is used to date organic materials but its presence can also help in understanding the age of groundwater when it has come into contact with organic matter. Since carbon-14 is produced naturally in the atmosphere and is absorbed by living organisms, it can provide information about the time elapsed since the water last interacted with the biosphere.

The other options would not be suitable for age dating groundwater in the context provided. For instance, while uranium and lead are used in dating geological materials, their time scales are much longer and are typically related to igneous rocks rather than groundwater. Oxygen and hydrogen isotopes are often involved in the study of water's source and movement rather than determining the age. Carbon and nitrogen isotopes do not have significant applications for groundwater age dating.