Testing subterranean water for a hazardous waste material repository

A method, comprising:

• determining a concentration of a krypton isotope in a subterranean water sample collected from a subterranean formation;
• determining that the concentration of the krypton isotope is less than a threshold value;
• and
• based on the determination, determining that the subterranean formation is suitable as a hazardous waste repository.

A method, comprising:

• determining a neutron flux of a first isotope in a subterranean formation;
• calculating, based at least in part on the determined neutron flux of the first isotope, a predicted production rate of a second isotope in the subterranean formation;
• calculating a first ratio of the predicted production rate of the second isotope relative to a theoretical production rate of a stable form of the second isotope;
• measuring respective concentrations of the second isotope and the stable form of the second isotope in a subterranean water sample from the subterranean formation;
• calculating a second ratio of the measured concentration of the second isotope relative to the measured concentration of the stable form of the second isotope;
• and
• based on a comparison of the first and second ratios, determining that the subterranean formation is suitable as a hazardous waste repository.