Techniques for testing subterranean water for one or more radioactive isotopes for a hazardous waste material repository include collecting, from a test drillhole formed from a terranean surface to a subterranean formation, a subterranean water sample from the subterranean formation; determining, with an accelerator mass spectrometry (AMS) system, a concentration of a radioactive isotope of an element in the subterranean water sample relative to a stable isotope of the element in the subterranean water sample; comparing the determined concentration of the radioactive isotope of the element in the subterranean water sample with a concentration of the radioactive isotope of the element in a surface water sample relative to the stable isotope of the element in the surface water sample; and based on the determined concentration of the radioactive isotope in the subterranean water sample being a specified percentage of the concentration of the radioactive isotope in the surface water sample, determining that the subterranean formation is a hazardous waste storage repository.