Crawford Hydrology Laboratory

What is a fluorescent dye?

Fluorescence is a physical property of certain compounds, including some dyes. In fluorescence, light photons of particular energy wavelengths are absorbed by the compound, then emitted at a lower energy level (longer wavelength) (Kass 1998, Rendell 1987).

We most commonly encounter fluorescence when looking at objects lit by a black light. Black lights give off energy in the 300-400nm range. Any material that absorbs light in this range will appear fluorescent. Optical Brighteners fluoresce under black light. They are commonly used in paper, fabrics, and detergents to make whites appear whiter because of this fluorescent property. Tinopal CBS-X optical brightener is one dye used by CHL that will fluoresce under black light. Most dyes used in groundwater tracing fluoresce optimally in light wavelengths above those given off by a black light.

Fluorescent dyes used by CHL absorb light between 300-600 nm and emit light at wavelengths about 15-20 nm longer than their absorbance maxima. CHL analyzes for fluorescent dyes using a Shimadzu RF5301 PC spectrofluorophotometer, which synchronously scans an excitation and emission range. We use an offset of 15 nm for water sample. If the instrument is outputting an excitation wavelength of 500 nm, it is simultaneously measuring the emission wavelength of 515 nm from the sample.
Why Use Fluorescent Dyes?

Fluorescent dyes make excellent tracers for a variety of reasons, including:

• They are water soluble.
• They are highly detectable (some below 10 parts per trillion).
• They have extremely low toxicity ratings.
• They are inexpensive.
• They are fairly stable in a normal water environment.
• They have been used successfully for tracing for over one hundred years.

Our fluorescent dyes are typically used in small amounts resulting in low concentrations in water due to their excellent detectability. Even at high concentrations, these dyes maintain their extremely low toxicity ratings.

Groundwater tracing often occurs at sites where the groundwater is contaminated and deemed unsafe to drink. The addition of a small quantity of dye into this water in order to map out its flow routes does not damage the water quality in any appreciable manner.

Groundwater tracing using fluorescent dyes is the best method of determining the direction and velocity of groundwater flow, determining the connectivity of recharge and discharge points. Dye tracing is therefore a vital tool in understanding groundwater flows, and the resulting information is used in a variety of ways to protect our groundwater and surface water quality.

Kass, Werner. 1998. Tracing Technique in Geohydrology. Stuttgart, Berlin. 581 pages.

Rendell, David. 1987. Fluorescence and Phosphorescence. Analytical Chemistry by Open Learning. Great Britain