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).
Groundwater tracing from an in-cave injection point.