5.3 Water Tracing Tests

Water-tracer tests are perhaps the most cost- and scientifically-effective method employed in the investigation of karst aquifers. Although naturally-occurring chemicals and isotopes are often, and increasingly, used as tracer agents, especially for studies of recharge and mixing dynamics, many tracer tests are conducted by injecting an artificial tracer—a substance that does not occur naturally in the water or occurs at negligible concentrations—and using various techniques to detect its presence and/or concentration, as it moves through the karst aquifer and resurges from the subsurface (Taylor and Doctor, 2017). The artificial tracer introduced can be a dye (usually a non-toxic fluorescent dye as shown in Figure 51), a dissolved chemical (generally a harmless salt), or particles (size and properties dependent on the nature of the aquifer and the property of interest). Any artificial tracer introduced must be considered toxicologically safe and sometimes a permit is required to introduce a tracer into the natural system and for access to place measurement devices to determine when and if the tracer arrived from the injection location to the monitoring location (Table 6).

Photographs showing tracer injection

Figure 51  Photographs showing injections of two commonly used fluorescent dye tracers: a) injection of sodium fluorescein into storm runoff drained by an open sinkhole throat; and, b) injection of rhodamine WT into the perennial flow of a disappearing stream. Photographs provided by Taylor (2021e).

Table 6  Summary of artificial tracers. Tracers classified as toxicologically “safe” by Behrens and others, (2001) are marked with an asterisk; the others were either not assessed or have associated toxicological concerns. Chemical Abstracts Service Registry Numbers (CAS RN) allow unambiguous identification. DOC: dissolved organic carbon, ICP MS: inductively coupled plasma mass spectrometry, IC: ion chromatography. From Goldscheider and others (2008).

Group Tracer name (CAS RN) Detection Limit General Problems Specific Problems
Fluorescent dyes Uranine (518‑47‑8) 10-3 μg/L Sensitive to light and strong oxidants. Analytical interferences between fluorescent dyes of similar optical properties.
Eosin (17372‑87‑1) 10−2 μg/L
Amidorhodamine G (5873‑16‑5)
Sulforhodamine B (3520‑42‑1) Ecotoxicological concerns
Rhodamine WT (37299‑86‑8) Genotoxic
Pyranine (6358‑69‑6) Biodegradable
Naphthaionate (130‑13‑2) 10−1 μg/L Analytical (optical)
Tinopal CBS-X (27344‑41‑8) interference with DOC
Salts Sodium Using Inductively Coupled Plasma-Mass Spectrometry (only cations) 10−3 to 0.1 μg/L and using Ion Chromatography 0.1 mg/L Sorption of cations
Variable and sometimes high natural background, particularly for Na and Cl
Bromide Can form toxic compounds
Iodide Bio-chemically unstable
Particles Fluorescent microspheres Detection of single particles Analysis is relatively time consuming.
Prone to filtration.
Bacteria Limited stability (inactivation)
Analysis within 24 hours

There are two main types of tracer tests, one is called a qualitative, or point-to-point tracer test, and the other a quantitative tracer test. Qualitative, point-to-point, tracer tests, are typically conducted using a variety of fluorescent dyes and have been the mainstay of karst hydrologic studies for many decades because of their relatively low cost and ease of implementation. These types of tests are generally conducted to determine karst groundwater flow directions and basin boundaries by helping identify the presence of one or more subsurface flow path connections between a dye-injection site (usually a sinkhole, sinking stream, or well), and downstream monitoring sites, such as a spring, surface stream, or water well. These tests are also commonly employed within caves to identify hydraulic connections and flow paths between cave segments or levels (Goldscheider and Drew, 2007). Hydraulic connections can be verified with visual confirmation if using colored fluorescent dyes or through the deployment of passive detectors, often informally called dye “bugs”. Bugs are composed of flow-through mesh bags containing activated coconut charcoal and deployed in a selected well, stream, or spring sites (Figure 52). The bugs absorb the dye and can be deployed for days or weeks before retrieval and analysis. These qualitative tests are almost always performed before a quantitative tracer test is conducted.

Photographs of dye absorbent material

Figure 52  Photographs of J. Van Brahana setting a bug (dye absorbent material in a flow through mesh bag) at Tree Spring at the Savoy Experimental Watershed, Arkansas, USA. The bug is fixed in place (usually for days or weeks) to determine if dye passes that location, but the exact time of arrival cannot be determined. Photograph by Kuniansky (2017)

If bugs are retrieved and replaced with new bugs sub-daily, daily, or at weekly intervals, the approximate number of days or weeks for the dye to reach the outlet can be estimated. After bugs are collected in the field, and washed, samples of the charcoal are treated with a basic-alcohol eluant to expel any adsorbed tracer dyes, which are identified by running an aliquot sample of the resulting elutant through a fluorometer or spectrofluorophotometer. Various types of fluorometry instruments can be employed (an eluant is the liquid used to remove dye from the activated carbon bugs and results in the elutant, which is the dye dissolved in the liquid eluant). An example of the graph resulting from spectrofluorophotometer analysis of an elutant is shown in Figure 53. The graph shows wavelength on the x-axis and relative excitation fluorescence on the y-axis for one sample from a given time.

Graph of an activated carbon sampler elutant

Figure 53  A graph of an activated carbon sampler elutant containing fluorescence peaks from pyranine, fluorescein, eosine, and rhodamine WT from a groundwater tracing study in British Columbia, Canada, from Aley (2002). A spectrofluorophotometer graph is a plot of wavelength on the x-axis and relative excitation fluorescence on the y-axis for one sample from a given time thus it identifies substances in the elutant. It is not a time series.

Quantitative tests can provide detailed information such as hydraulic conductivity and contaminant travel time for a conduit flow system, but are generally less commonly conducted than point-to-point tracer tests due to the need for more intensive monitoring and dye-analysis procedures. Quantitative tests require information on the amount of dye injected and the time series of concentration and groundwater discharge rate from the exit point (a spring or well). With fluorescent dye tracers, a field fluorometer and data logger typically are calibrated with standards in the field according to the manufacturer’s instructions, and the equipment is installed at the discharge location. Grab samples are collected frequently and the grab samples are analyzed in the laboratory using a laboratory calibrated spectrofluorophotometer to verify the field fluorometer results. It is always advisable to collect and analyze grab samples even when a field fluorometer/data logger is deployed.

An example normalized breakthrough curve from a quantitative test in the Savoy Experimental Watershed at the University of Arkansas in the USA is shown in Figure 54. A normalized graph is created by taking the maximum concentration and dividing time series concentration values by that concentration such that two different tracer breakthrough curves can be plotted on the same graph with the y-axis ranging from 0 to 1 and elapsed time starting at zero for the time the tracer was injected on the x-axis. For the tracer test shown in Figure 54, there was one sinking stream where a slug of dye and salt (NaCl) were injected and two springs that were known to be outlets of this sinking stream.

Graph of dye concentration

Figure 54  Normalized Rhodamine WT dye and chloride tracer-breakthrough curves for Copperhead and Langle Springs, Savoy, Arkansas, USA, April 13 to 17, 2001. Elapsed time begins at zero which is the time that tracer was introduced at noon on April 13. Chemical data collection began shortly after injection. Illustration provided by J.V. Brahana on November 13, 2017 from a University of Arkansas student, Tiong Ee Ting.

For successful quantitative water-tracer tests, it is critical to:

  • consult local geology experts when selecting injection and discharge location(s);
  • collect background samples of water to determine if there is background fluorescence or chemical that may interfere with the artificial tracer to be injected;
  • select the dye, solute or particulate that is appropriate for the hydrogeologic setting and the study objective;
  • use regression equations, once the appropriate dye is selected, to estimate the amount of dye required (Field, 2003; Worthington and Smart, 2003);
  • run point-to-point tests to confirm the link(s) from the sink(s) to the discharge location(s);
  • determine which method to use for collecting the time series of discharge and concentration (or fluorescence) data;
  • determine the thickness of the unsaturated zone and if the tracer will be placed, rather than injected, into a sinking stream or well, determine whether a tanker of deionized water is required to flush the tracer into the karst system;
  • determine whether a pulse of tracer will be used or it will be continuously injected; and,
  • for dyes, use dark sample bottles, or have a dark place to store the grab samples, because many organic dyes will lose fluorescence if exposed to sunlight.

Exercise 19 invites the reader to explore the relative behavior of the dye and the salt shown in Figure 54.

Of the eight fluorescent dyes, the five most useful are: Eosin (Eos), Uranine (Sodium Fluorescein) (Fl), Pyranine (Py), Rhodamine WT (RWT), and Sulforhodamine B (SRB), because all of these dyes are anionic compounds and thus, less subject to adsorption onto clays and similar materials than cationic dyes (Aley, 2002). Each of these five dyes has distinct characteristics and wavelength (for example, color) and are safe at low concentrations, but some may be better suited to one environment or another. For example, Uranine (fluorescein) is bright green, very soluble, safe and has a low detection limit, but will degrade in acidic or organic-rich waters. If one is trying to determine the outlet for several sinking streams and several springs in the same area, dyes of differing optical properties must be used at each sinking stream location to minimize optical interference in analysis of elutant from bugs used in a point-to-point test or grab samples of each spring. A spectrofluorophotometer analysis of one sample from a dye tracer study where four dyes made it to the outlet bug is shown in Figure 53. To summarize the use of fluorescent dyes as tracers, Taylor and Doctor (2017) write: “Like any other hydrogeologic investigation technique, dye-tracing tests require good planning and implementation, proper interpretation of data, and understanding of the potential limitations and uncertainties associated with the technique. Dyes should be selected for conservative transport behavior, negligible toxicity, and unambiguous detection at low concentrations (Taylor and Greene, 2008). Successful use of dyes requires: (1) preinjection monitoring for substances present in the water that might interfere with detection of dye, (2) determination of the proper amount of dye to inject for the purpose of the test, (3) choosing the appropriate methods for dye monitoring and detection, (4) selection of sites to be monitored, and (5) determination of the duration of the test monitoring period. It is important to also consider the field or hydrologic conditions occurring at the start and over the duration of each dye-tracing test because groundwater flow directions and velocities can change drastically in karst between low-flow and high-flow conditions. These topics are discussed in detail by Smoot et al., (1987), Käss (1998), Goldscheider and Drew (2007), and Taylor and Greene (2008), among others.

Of the salts shown in Table 6, common table salt (sodium chloride, NaCl) is often used because it is inexpensive, and its concentration can be sensed by measuring specific conductance. The water-soluble salts split into cations and anions that increase electrical conductivity of water. Some samples can be chemically analyzed for the cation and/or anion concentration and compared with a time series of electrical conductance of the water (which is inexpensive relative to chemical analysis). Goldscheider and others (2008) explain: “More specific chemical analysis allows use of lithium, potassium and strontium as cationic tracers, while bromide and iodide can be used as anionic tracers. Anions are generally more conservative than cations, which are prone to cation exchange and, thus, retardation. Variable natural background concentrations limit the use of salt tracers, and high concentrations may be harmful to biota.

Non-toxic microspheres or particles are sometimes used in tracer studies for understanding of the transport of pathogens in the karst system (Harvey, 1997; Mahler et al., 1998 and 2000; Auckenthaler et al., 2002; Göppert and Goldscheider, 2008; Harvey et al., 2008).

Exercise 20 invites the reader to reflect on what a spectrofluorophotometer measures.

For readers that are interested in learning more about conducting tracing tests in karst, the following references are suggested: Behrens, 1986; Mull and others, 1988a; Aley, 1997, 2001 and 2002; Behrens and others, 2001; Field, 2002 and 2003; Worthington and Smart, 2003; and Goldscheider and others, 2008. A partial list of investigative reports that discuss applications of tracer testing include: Mull and others, 1988b; Mull, 1993a and 1993b; Bayless and others, 1994; Robinson, 1995; Pavlicek, 1996; Taylor, 1997; Kidd and others, 2001; Spangler and Susong, 2006; Kozar and others, 2007; Long and others, 2012; Spangler, 2012; Gouzie and others, 2014; and Kuniansky and others, 2019.


Introduction to Karst Aquifers Copyright © 2022 by Eve L. Kuniansky, Charles J. Taylor, and Frederick Paillet. All Rights Reserved.