Leaks in underground industrial pipes can be found by adding radioactive material to the liquid in the pipe.
A Geiger-Muller tube ( GM tube ) is moved along the ground where the pipe is buried.
The count rate will be found to increase where the pipe is leaking as a larger amount of liquid containing the radioactive source will collect there.
The radioactive source would be gamma* emitter because it can penetrate through the soil (and metal pipe) and be detected at the surface.
The gamma emitter would have a short half-life to minimize the possible danger to environment.
*Alpha particles are easily absorbed by a few centimetres of air whereas beta particles may be absorbed by a few millimetres of aluminium.