Rhodamine can indicate how water moves and monitoring rhodamine 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).
Rhodamine WT (Water Tracing) is a good tracer as it is water-soluble, highly fluorescent, inexpensive and reasonably stable (one of the most conservative 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.
Rhodamine is often used to measure the time of travel (TOT) for surface waters, groundwater and wastewater (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.