Use of Fluorescent Dyes for Groundwater Tracer Studies
Tracers have been utilized for a number of years to provide insight into various aspects of groundwater hydrology. In general, tracers come in two types. Environmental tracers are those that are inherent components of the water cycle (e.g., stable and radioactive isotopes), while artificial tracers are actively injected into the system of investigation. You may be familiar with artificial visible tracers (e.g., brilliant blue dye), however, these tracers often are used for purposes other than groundwater field study applications. Fluorescent dyes is one group of artificial tracers which has routinely been applied in the field for groundwater studies and for which a wealth of technical literature exists.
Fluorescent dyes are popular in the environmental industry because of their low/non-toxicity to the environment and potential receptors, relative ease of handling and application, low detection limit, high sensitivity of analysis, and relatively low cost. In addition, fluorescent dyes move at the speed of groundwater (or slightly faster) and only small quantities are required to perform most groundwater tracer studies (approximately 1 to 10 pounds). Unlike visible tracers, fluorescent dyes require laboratory analysis of the groundwater samples for proper detection and evaluation. Although there are numerous fluorescent dyes available, four are listed below for brevity. The common names of the fluorescent dyes can vary by manufacturer or retailer, therefore, the chemical abstract service (CAS) and color index (CI) are provided:
• Eosine (CAS 17372-87-1; CI 45380);
• Fluorescein (CAS 518-47-8; CI 45350);
• Pyranine (CAS 6358-69-6; CI 59040); and
• Rhodamine WT (CAS 37299-86-8; CI not assigned).
The fluorescent dyes listed above are anionic compounds, which means that they are less likely to adsorb to clays and other materials when injected into the subsurface. In addition, they can be amenable to use in both bedrock and unconsolidated aquifers. Some potential groundwater hydrology applications of these fluorescent dyes may include, but are not limited to:
• evaluation of flow paths;
• discharge measurement;
• hydraulic connections;
• flow and transport parameters;
• modeling calibration;
• residence time;
• mixing processes;
• surface water interaction;
• contaminant transport; and
• engineering hydrology.
Like many other characterization methods used in the environmental field, fluorescent dye tracer studies should be evaluated on a site-specific and application-specific basis. Cox-Colvin recommends several factors be considered prior to performing a groundwater tracer study with fluorescent dye. The appropriate type and quality of fluorescent dye should be selected, taking into account background and interference fluorescence, as well as site hydrogeology. The appropriate injection location(s), injection radius of influence, and injection procedures should reflect the goals of the study. A sampling methodology should be developed, which can include a combination of groundwater samples and activated carbon accumulation samplers. Quality control procedures are important to ensure that no fluorescent dye is lost or destroyed during sample collection and analysis, and the potential for cross contamination is minimized. An analytical laboratory should be identified that has appropriate instruments, analytical methods, and ability to distinguish among fluorescent dyes, and background and interference fluorescence. Many laboratories that provide traditional groundwater sample analyses for contaminants of concern do not have these capabilities. Finally, injection of fluorescent dyes through a well or direct push tooling can be considered a Class V injection well, which will require specific information be submitted to the regulatory agency prior to (and possibly after) injection work.
Nick M. Petruzzi, PE is a project manager at Cox Colvin & Associates, Inc. Mr. Petruzzi holds degrees in both geology and environmental engineering. He evaluates and designs both established and innovative remedial alternatives for the treatment of contaminated soil and groundwater at industrial facilities under various regulatory programs and project complexities/scales. Both the evaluation and design of remedial alternatives by Mr. Petruzzi regularly incorporate green and sustainable remediation. Mr. Petruzzi is an active member in the Interstate Technology and Regulatory Council (ITRC) Green and Sustainable Remediation team, the Ohio Society of Professional Engineers (OSPE), and the National Ground Water Association (NGWA).