Dye Tracing FAQS

Fluorescent dye tracing is used to study the movement of groundwater. Dye trace investigations are designed to solve problems involving the origin, destination, routing, and velocity of groundwater flow.  This is typically the first step for obtaining information for aquifer monitoring, pollution prevention, or water resource management and development.

Prior to a dye trace, a karst hydrogeologic inventory is conducted of all relevant karst features in the study area. This is important in order to help identify dye injection and dye monitoring locations and to assure that critical locations are not overlooked. During the dye trace process, a fluorescent dye is injected into the subsurface via a well, sinking stream, sinkhole, or excavation pit. The route of the dye (and hence the groundwater) is determined by placing charcoal receptors or taking water samples at groundwater resurgence points such as karst windows or springs. Dye tracing is also applicable in some non-karst areas and wells are often used for dye injection and sampling across various geologic settings.

Dye tracing has advanced rapidly in the last few decades, partly due to the development of charcoal dye receptors for monitoring. Charcoal receptors are submerged in the water and as water travels over the receptors the dye, if present, adsorbs to the surface of the charcoal. The receptor is then collected and processed in the lab where a chemical solution is used to remove the dye from the charcoal. Water samples can also be collected at the monitoring locations and analyzed for the presence of dye.

The spectrofluorophotometer using synchronous scanning measures the wavelength of light emitted from the sample. As each dye has a unique emission wavelength it is possible to distinguish multiple dyes in one sample which allows for multiple dye injections to be performed in the same area for complex studies. Fluorescent dyes can be detected at very small concentrations (less than one part per billion).

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 RF 6000 or RF 5301 PC spectrofluorophotometer, which synchronously scans an excitation and emission range.  

Groundwater tracing using fluorescent dyes is the best method to determine the connectivity of recharge and discharge points and allows for determination of direction and velocity as well. Dye tracing is a vital tool in understanding groundwater flow and the resulting information is used in a variety of ways to protect groundwater and surface water quality.

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 natural waters.
• They have been used successfully for tracing for over one hundred years.

 The answer to this dye tracing question depends on:

• Laminar or turbulent flow of water
• Number and type of injection locations
• Background fluorescence
• Water quality parameters

We recommend background fluorescence analysis for every monitoring location, which must be done prior to the dye injection. Background monitoring helps to determine the types and amounts of dye that work best for your site by revealing the any background fluorescence of the water at your monitoring sites. Water will often have some level of fluorescence from natural or man-made sources. When multiple injections are planned, we choose dyes that have very different wavelengths of fluorescence. For example, fluorescein and sulphorhodamine B are good “companions” for dye trace investigations since their fluorescence wavelengths are 510 nm and 582 nm in water, respectively.

Crawford Hydrology Laboratory uses the following dyes:

• Tinopal CBS-X (Optical Brightener)
• D&C Green 8
• Fluorescein (Uranine C, Acid Yellow 73)
• Eosine (Acid Red 87)
• FD&C Red 3
• D&C Red 28 (Phloxine)
• Rhodamine WT
• Sulphorhodamine B (Acid Red 52)