FAQ
1. Which fluorescent dye(s) should I use?
2. How much dye should I use?
3. Can you detect more than one dye in a sample?
4. Should I use charcoal dye receptors or water samples?
5. How many sites should I monitor?
6. Is dye purchased from one company equivalent to that from another company?
1. Which fluorescent dye(s) should I use?
The dyes that work best for a project depend on:
- Laminar or turbulent flow of water
- The number of injection locations
- Background fluorescence present
- If the water will be exposed to sunlight
- Extremes in water pH
- If a visual trace, or an invisible trace, is necessary
We recommend a background fluorescence sample analysis for every monitoring location, which must be done prior to the dye injection. When a background analysis is performed before choosing the dyes, it will help to determine the types and amounts of dye that work best for your site by revealing the spectrum for 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.
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2. How much dye should I use?
There has been much debate on this subject and there are formulas to help determine the amount of dye needed. However, there is no easy answer, since the travel times and dilution factors are largely unknown, despite our best estimates. Never-the-less, 1-10 lbs of dye will suffice for most dye traces. We will make recommendations for each situation based on distance to monitoring sites, the site’s rock strata or aquifer characteristics, expected travel times, type of injection site (well, sinkhole, sinking stream), and our past experience.
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3. Can you detect more than one dye in a sample?
Yes. Each dye we sell has a unique wavelength of fluorescence. We can uniquely identify each dye, and can identify multiple dyes in a sample. Dyes with similar wavelengths may have fluorescent peaks that overlap. In these cases, peakfitting software is necessary. We use this program conservatively to estimate the peak location and concentration of dyes with interfering peaks. However, a high concentration of one dye can swamp out the peak of a dye with a very low concentration.
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4. Should I use charcoal dye receptors or water samples?
Using activated charcoal packets, installed in a well, stream, or spring is the best method for determining point-to-point connections of groundwater flow, and also gives information concerning the time of travel of the dye. Using charcoal dye receptors is the best and most cost-effective method to determine if two points are hydraulically connected. Fluorescent dyes are adsorbed onto the charcoal over time, and can be detected from eluted charcoal at lower concentrations than in water. However, charcoal can not quantitatively assess the amount of dye in the water. Dye receptors can be exchanged (receptor collected for analysis and replaced with new receptor) at varying frequencies. Commonly exchanges are done at a frequency of every one or two weeks, though they may be done as frequently as a few times a day or as infrequently as every few weeks or months. However, we recommend a maximum of a few weeks.
Analysis of water samples for fluorescent dyes gives an accurate quantification of the dye concentration in the water at a given time. Using periodic sampling, especially with an automatic water sampler, provides precise information about the timing and concentration of the dye as it moves through the system in the form of a dye break-through curve. This information is valuable for detailed hydrologic studies, and is valuable in potentially contentious dye trace investigations.
You may wish to use a combination of charcoal dye receptors and water sampling for your dye trace investigation. Even if you plan to use only charcoal receptors, those purchased from CHL include a water sampling vial, which should be filled at the time of receptor collection as a back-up. The water sample can be held at the lab, and analyzed only if needed.
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5. How many sites should I monitor?
In order to not miss the dye, it is necessary to monitor every possible resurgence point between the injection location and a major discharge boundary, such as a river. Sometimes project managers will choose to monitor only the most likely resurgence points. This method requires less time and cost, and may provide all the information desired. However, some or all of dye resurgence points may be missed, and a repeat of the dye trace may be necessary.
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6. Is dye purchased from one company equivalent to that from another company?
Maybe. Most dyes are not made for laboratory or water tracing applications. Dye manufacturers produce products with varying purities and strengths. The specifications vary from manufacturer to manufacturer, and distributor to distributor. Additionally, the same dye can have many names. One dye may be cheaper than another because it is actually less pure. For example, a “green dye” may look similar to fluorescein, but it may fluoresce at a different wavelength than the fluorescein CHL sells.
CHL has obtained the most reasonably priced quality bulk dyes with the highest purity and strength. We test each batch of dye received to ensure quality and check for cross-contamination problems. We carry the same dye products that we use to make our laboratory standard solutions, or we cross-check the products against our standard solutions. We know the fluorescence emission wavelengths of the products, and their purities.
You can use dyes purchased from other companies. On a CHL lab analysis report sheet, the concentrations (parts per billion) of dye in a sample will be reported in units equivalent to the dyes we have in stock.
If you obtain powder dyes, these should be mixed with water before getting to the injection location. CHL does not recommend injecting powder dye, due to a high probability of cross-contamination in the field. CHL dyes are mixed to the solubility point, placed in a jug and mailed to you, for ease of use and clean-up, and minimization of cross-contamination risk.
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References:
Kass, Werner. 1998. Tracing Technique in Geohydrology. Stuttgart, Berlin. 581 pages.
Rendell, David. 1987. Fluorescence and Phosphorescence. Analytical Chemistry by Open Learning. Great Britain