Dye tracing is used to study the movement of groundwater. 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 a well, sinking stream or sinkhole. The route of the dye (and hence the groundwater) is determined by placing charcoal receptors or taking water samples at karst windows, springs, or other karst and surface water features identified from the inventory.
Dye tracing of karst groundwater has advanced rapidly in the last few decades, partly due to the development of charcoal dye receptors for monitoring. Receptors are submerged in the water and the charcoal adsorbs the dye as the water travels over the receptor on its way to the spring, karst window, or cave. The receptor is collected and taken back to the lab for analysis where a chemical solution is used to remove the dye fom the charcoal. Water samples can also be taken at the monitoring locations and analyzed for the presence of dye.
An instrument that measures the wavelength of the beam of light emitted from the sample is used to detect and uniquely identify the dye. This is possible because different colored dyes fluoresce at different wavelengths, 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).