The water seeping into shallow workings or shafts may be traced to the surface source by means of tracer dyes or salts. These substances, however, may be leached out of the water by the soil or strata. Some radioactive isotopes are better tracers because of the high sensitivity with which they can be detected. Tritium, an isotope of hydrogen, is unique because it can be used to label the actual water molecule to be traced and is not chemically removed by the strata.