A substance that is used to follow the passage of groundwater in places where it cannot be observed directly. Typical tracers include fluorescent dyes and salt. The presence of radioactive isotopes, e.g. tritium and carbon-14, may also be used as tracers in that they allow the age of groundwater to be determined. The presence of small amounts of other substances may also be used to make deductions about the origin and flow path of groundwaters.