For instance, leaks in underground water pipes can be discovered by running some tritium-containing water through the pipes and then using a Geiger counter to locate any radioactive tritium subsequently present in the ground around the pipes.
(Recall that tritium is a radioactive isotope of hydrogen.) Tracers can also be used to follow the steps of a complex chemical reaction.
Every chemical element has one or more radioactive isotopes.
For example, hydrogen, the lightest element, has three isotopes, which have mass numbers 1, 2, and 3.
Radioactive isotopes of radium, thorium, and uranium, for example, are found naturally in rocks and soil.