Fluorescein can indicate how water moves, and monitoring fluorescein in water can be used to trace pollutants, study dispersion dynamics and aeration and other water flow studies. The use of fluorescent dyes allows detection at much lower levels than other tracers (e.g. microbes, radionuclides, particles).
Fluorescein is a good tracer as it is water soluble, highly fluorescent and inexpensive. However, it is pH sensitive – the relative fluorescence falls below pH 7 and falls significantly below pH 6.0. Fluorescein is also photosensitive – direct sunlight can destroy the dye molecule leading to almost total loss of the signal within hours. With a temperature coefficient of -0.36%/℃, fluorescein is one of the most temperature-stable tracer dyes.
Fluorometers can detect low levels of fluoresced light and amplify the electronic signal allowing very low dye levels to be detected – sub parts per billion (PPB). The amount of fluorescence detected is relative to the concentration of the dye in the water, so this measurement can be used to give an accurate indication of the tracer dye levels in the water sample. Note, at very high dye concentrations, multi-photon events can result in a non-linear relationship. In practice this is rarely encountered due to the dilution effect when e.g. a few litres of dye are added to a waterbody. In-situ fluorometers have rapidly replaced in-vitro water sampling as there is a reduced requirement for trained personnel (and the potential for human error) and because near-real time results are possible.
Fluorescein is often used to measure the time of travel (TOT) for surface waters, groundwater and waste water (the movement of tracers from one point to another e.g. breakthrough studies in karst rock) where a concentrated solution of the dye is injected at a specific point and the diluted levels are detected by a fluorometer at a point downstream.