Rhodamine is a red dye which measures the TOT (Time of Travel) for surface and ground water. It indicates how water moves, and is useful for tracing pollutants, studying aeration and dispersion, as well as waste buildup and flushing in estuaries and bays and waste water retention and flushing in wetlands.
Rhodamine fluoresces when irradiated with light of a particular wavelength. For field measurements, in-situ fluorometers induce rhodamine to fluoresce by shining a beam of light of the proper wavelength into the water and then measuring the higher wavelength light which is emitted.