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Water tracing in karst aquifers
Chapter 134
Water tracing in karst aquifers William K. Jones Karst Waters Institute, Leesburg, VA, United States
Definitions and objectives of tracer tests in karst Water tracer tests are usually conducted to establish the hydrologic connections between two or more points. The tracer is an identifiable label or marker added to flowing water that establishes the links between the injection point of the tracer and the monitoring points where the tracer reappears. Fluorescent dyes are the most commonly used tracers in karst aquifers, but a wide range of substances has been used successfully. The experimental design of a tracer test may be qualitative to simply establish if a hydrologic connection exists between two points, or quantitative to measure the time-concentration series (breakthrough curve) generated by the recovery of the tracer. Water tracer tests usually work well in karst areas because of the fast groundwater flow rates and the prevalence of flow paths restricted to discrete conduits. The level of effort and the cost of conducting a tracer test is a function of the question being asked. Many tracer tests are an extension of cave exploration and are conducted to “see where the water goes.” The tracer test may have the relatively simple objective of establishing a connection between two cave passages separated by an impassable reach. Tracer tests may be used to determine the destination of water flowing into a sinkhole or blind valley. The karst drainage basin contributing water to a spring or resurgence may be estimated based on the results of tracer experiments. The interbasin transfer of water is frequently demonstrated by tracer tests. The travel rate of a tracer may be used to establish some of the hydrologic properties of an aquifer. Examination of the breakthrough curve of a tracer test can aid in the interpretations of some of the internal flow characteristics of the aquifer. The movement of a tracer may mimic the subsurface behavior of a pollutant introduced into the aquifer at the same point. Tracers may be used to identify the source of pollution at a spring. Tracer tests are sometimes used to demonstrate the “vulnerability” of karst aquifers to chemical spills and pollution associated with inappropriate land use practices and to establish “wellhead protection areas” for public water supplies. A study of the Unica River basin in Slovenia (Gabrovsek et al., 2009) provided a demonstration of the range of hydrologic information that can be obtained from tracer tests and the applicability to the protection and management of karst water resources. Naturally occurring chemicals or isotopes may be used to determine the age or residence time of water at sampling points within the aquifer. The sources of the water in terms of percentages contributed by conduit flow versus drainage from the overlying epikarst or deeper fracture zones may be studied. These chemical markers are considered tracers, but they identify the various components or storage areas of the aquifer rather than establish direct hydrologic connections or paths between specific points in the aquifer.
History of water tracing in karst aquifers A considerable body of folklore concerning underground water connections has accumulated throughout the world’s cave areas. Tests have been reported using tracers such as wheat chaff, duck feathers, marked logs, tagged eels, and muddy water from storm events. A 2000-year-old tracer test is attributed to tetrarch Philippus who supposedly established the source of the springhead of the Jordan River in 10 AD. Chaff was thrown into Phialo Pond (Berekhat Ram Crater Lake) and reemerged from the cave spring Panium, the head of the Jordan River. Although the results of this test have been questioned, it does establish that the basic idea of tracing underground water is quite old. A scientifically planned tracer test was conducted in 1872 by A. Hagler to determine the origin of a typhoid fever outbreak in the village of Lausen in Switzerland. About 800 kg of salt (NaCl) was injected in a sinking stream on a farm south of the village and the water supply spring for the village showed a strong reaction for chloride the following day (K€ass, 1998). The start of modern water tracing studies followed the discovery of fluorescein by A. V. Baeyer in 1871. The sodium salt of fluorescein, called uranine in Europe, rapidly became the most used and probably the most successful groundwater tracer for cave 1144
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Water tracing in karst aquifers Chapter 134
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FIG. 1 Fluorescein sodium powder is red but turns bright fluorescent green when dissolved in water. Here the dye is being added to an underground stream just upstream of an impassible sump. (Photo by Philip C. Lucas. Used with permission.)
and karst studies to this day (Fig. 1). The first reported use of sodium fluorescein as a tracer was an experiment conducted in southwestern Germany on October 9, 1877, by Professor Albert Knop. Ten kilograms of sodium fluorescein supplied by C. Ten Brink was injected at infiltration points in the bed of the Danube River and reappeared 2 days later and 12 km away at the Aach Spring, a tributary of the Rhine River. A test conducted a couple of weeks earlier from the same location used almost 10,000 kg of salt and may have been the first quantitative tracer test with hourly sampling at the Aach Spring. Work on the development of methods for tracing underground waters continued in Europe. Many of the early tracer tests used large quantities of dye, because detection depended on visual coloring of the water at the resurgence. An interesting example of an accidental tracer experiment occurred in the French Franche-Comte in 1901. As the result of a fire at the Pernod distillery at Pontarlier, a quantity of absinthe poured into the Doubs River. A significant amount of the alcohol seeped through the bed of the river and reemerged the following day 10 km away at the source spring of the Loue River. The test of the connection between the Doubs and the Loue was repeated by E. A. Martel in 1910 using 100 kg of sodium fluorescein. The Loue was brightly colored 2 days later for a distance of 100 km downstream. The source of the Garonne River in southern France was determined by a tracer test in 1931 by Norbert Casteret. He injected 60 kg of sodium fluorescein at the Trou du Toro on the Spanish side of the Pyrenees and it emerged 3.7 km away and 10h later at the Goueil de Joueou on the French side. The Garonne River was colored a bright green for over 50 km downstream. Quantitative tests at the same site in the early 1990s used less than 2 kg of sodium fluorescein and yielded travel times for the tracers of about 11 h during high-flow conditions in the summer and 180 h for a low-flow test in the winter (Freixes et al., 1997). Various tracer techniques were tried in Europe through the 1920s. A “fluoroscope” was developed which allowed a rough estimation of dye concentrations in water samples by visual comparison with prepared laboratory standards. A number of different salts, dyes, and even radioactive compounds were tried. Bacteria were tried as a particulate drift tracer as early as 1896. Spores from the club moss Lycopodium calvatum were suggested as a possible tracer in 1910, and the first reported tracer test using spores is from Europe in 1940 (K€ass, 1998). Spores became a popular tracer in Europe in the 1950s because they could be dyed different colors and several sink-points could be traced simultaneously. Plankton nets were suspended at the springs to passively collect the spores, so constant surveillance of a number of springs was no longer required. Spores dyed with fluorescent dyes proved to be easier to identify under a microscope. Drifting tracers such as spores and bacteria are still used to a certain extent in Europe but have never been popular in North America. One novel tracing experiment in Slovenia in 1929 used marked eels. The dorsal fins of 494 eels were notched and the eels were released in the Reka River that sinks into Skocjanske Cave and resurges 34 km to the west at the Timavo Springs in Trieste Bay (Italy). Twenty-nine of the eels were caught in eel pots at the Timavo Springs during the 1-year observation period. The Yorkshire Geological Society carried out a series of tracer tests in the Ingleborough area of Great Britain in 1904. One principle from this work has been to test “key sinks” and to guess the remainder in lieu of testing everything. This approach is still practiced in many karst areas, but more intensive studies at later dates sometimes produce surprising results. Much of the subsequent work on tracing underground drainage in the British Isles has been conducted by caving groups associated with various universities. A study of the caves of northwest County Clare, Ireland, in the early 1960s by E. K. Tratman and the University of Bristol Speleological Society, involved a number of tracer tests using sodium fluorescein and passive carbon detectors to monitor the resurgences. Just as professional cave and karst studies in North America lagged behind Europe in general, so did the interest in tracing underground water. The first description of tracer tests from North America was a U. S. Geological Survey water supply paper by
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R. B. Dole in 1906. This paper discussed the use of sodium fluorescein as a tracer, but few tracer tests are reported from North America before the late 1950s. Whereas most of the water tracing studies in Europe were conducted by professional hydrologists or speleologists, often at considerable expense, the real beginning of karst water tracing studies in North America was the contribution of unpaid cavers as an extension of cave exploration. A brief description of a method to recover sodium fluorescein sorbed onto activated coconut charcoal granules was published by J. R. Dunn in a cave club newsletter in 1957. This discovery offered an inexpensive technique to monitor different resurgences without the necessity of constant surveillance. The researchers could place carbon packets (called “Dunn Bugs”) in the springs, inject the dye in a sinking stream or cave, and return in a few weeks to collect and test the carbon packets. The concentration of the dye on the carbon increases with the exposure time to the dye, so recovery concentrations of sodium fluorescein below the normal visual threshold could be detected. The testing relied on visual identification of the dye as a fluorescent yellow-green sheen floating on top of the carbon in an elutant. Apparently a very similar technique was described by Mayrhofer in 1904 (K€ass, 1998), but the activated carbon system was used in Europe more as a technique for enriching the dye concentration in a sample than as an unattended monitoring system. The first field test of the passive detector system in North America was described by T. D. Turner in 1958. Turner reported a tracer test using carbon detectors and 4 g of sodium fluorescein over a distance of 4.8 km near Pine Grove Mills, PA. Water tracing in North American cave studies really began in the early 1960s using sodium fluorescein and passive detectors of activated carbon. Hermine Zotter began a dye tracer study in Pocahontas County in east central West Virginia in 1960. Thomas Aley conducted a tracer test for a court case in California in 1963. This was probably the first use of underground water tracing to resolve litigation in the United States. Aley worked on a study for the US Forest Service in the Ozarks in Missouri in 1966 and conducted a 64-km long trace from the Eleven Point River Basin to Big Spring on the Current River Basin. William Jones conducted a study for the U.S. Geological Survey in Greenbrier County, West Virginia, in 1966. The results of the tracing tests were coupled with the positions of surveyed cave streams and topographic divides to delineate karst drainage basins. Charles Brown was conducting tracer tests in the Maligne basin in Alberta, Canada, by 1969. James Quinlan became the park geologist for Mammoth Cave National Park in Kentucky in 1973 and started an intensive series of water tracing tests. The results of the tracing tests were combined with data from cave surveys and potentiometric contours to produce a detailed map of drainage basins and subbasins around Mammoth Cave (Ford and Williams, 2007). The development in the early 1960s of fluorometers that control or measure light emissions in both the excitation and emission wavelengths of different fluorescent compounds enabled dye analytical methods to become much more quantitative. Quantitative water tracing using fluorometers dates from the early 1960s. Much of the early quantitative tracer work was for surface water timeof-travel studies. Fluorometers also allowed several different fluorescent compounds to be used simultaneously because the instruments could be set to narrow specific wavelengths. Much of the development of quantitative tracer techniques was presented in a series of conferences in Europe beginning with the Specialists Conference on Tracing of Subterranean Waters in Graz, Austria, in 1966. This conference series continued at about 4-year intervals as the Symposium on Underground Water Tracing (SUWT) conferences. The published proceedings from the series contain a wealth of information on various tracer techniques for both karst and porous media water tracing. Much of the material from these conferences is distilled in the book by K€ass (1998). A U. S. Geological Survey paper by Wilson (1968) and a paper on the evaluation of fluorescent tracers by Smart and Laidlaw (1977) were probably the most influential papers on quantitative water tracing published in North America.
Tracers Groundwater tracers are generally classified as: (1) naturally occurring (often accidental in their addition to the water), (2) artificial tracers deliberately introduced into the aquifer, and (3) pulses. Naturally occurring tracers include the chemical constituents and isotopes naturally found in water or present due to the activities of man. Tritium, fluorocarbons, pollutants from waste sites, and thermal waters may all serve as tracers in some circumstances. Isolated thunderstorms may generate a distinctive flow response at springs some distance from the storm. Organisms unique to a particular subsurface drainage basin may be considered a tracer if their distribution can be used to define the boundaries of the basin. Naturally occurring tracers are often used to “date” the water, artificial tracers are used to label a specific sink or cave, and pulses send an identifiable signal through the conduit or sometimes a large part of the aquifer. Most tracer tests in karst areas are conducted using artificially injected tracers. These tracers may be broadly classified as: (1) water-soluble tracers (dyes, salts), (2) particulate or drift material tracers (club moss spores), and (3) physical pulses (flood waves). The ideal tracer should be: 1. 2. 3. 4.
Nontoxic Not normally present in the study area (low background) Detectable at very low concentrations Conservative (minimal sorptive and decay losses)
Water tracing in karst aquifers Chapter 134
5. 6. 7. 8.
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Have the same density as water (or be neutrally buoyant) Relatively inexpensive Easy to sample using passive collectors Easy to analyze and quantify in the laboratory
The vast majority of tracer tests in cave and karst areas are conducted using fluorescent dyes. All fluorescent dyes have some tendency to adsorb to clay minerals and some have high photochemical decay rates. The relative fluorescent intensity of many dyes is pH and temperature dependent. These considerations are usually not a problem for underground tracing in cavernous aquifers with high flow rates. Tracer tests in diffuse-flow aquifers often involve the use of more conservative isotopes where the longer residence times make fluorescent dyes less suitable. A list of the principal fluorescent tracer dyes commonly used in North American karst studies is presented in Table 1. Tracer tests from Europe have largely been conducted quantitatively and required direct sampling of the resurgences. Studies in the British Isles and North America through 1990 have mostly been qualitative and used passive collectors to monitor the resurgences. Qualitative tracing is still used for reconnaissance type studies and work in remote areas, but quantitative tracing techniques are now the norm for most scientific studies throughout the world. The fluorescent dyes commonly used as tracers are believed to be quite harmless to people and the environment. Reviews of the toxicity data for tracer dyes were presented by Smart (1984) and Behrens et al. (2001). Several dyes in the rhodamine group, especially Rhodamine B, are not recommended due to environmental concerns, but the high sorptive tendency for some of these dyes make them a poor choice for groundwater tracing under any circumstances. A lot of confusion exists over the names of many of the tracers. Sodium fluorescein is known as uranine in Europe and commonly called fluorescein in the North American literature. Only the sodium salt of fluorescein is soluble enough to make it an efficient tracer. When describing a dye, the Color Index name should be cited at least once. Sodium fluorescein has a Color Index constitution number CI 45350 and a generic name of acid yellow 73. Also, all dye shipments should be checked when they are received to make certain the correct product is being used. Note that a few words of caution are in order for anyone planning on conducting tracer tests. Specialists now do most dye tracing, and water tracing by untrained people may be very damaging. In the United States, some states require a permit for conducting a tracer test, and the local water inspectors should always be notified in advance of any test so a hazardous materials team
TABLE 1 Principal fluorescent tracers used in North American karst studies Tracer Colour index name Color
Excitation Emission Maximum
Passive Detector
Remarks
Phorwite AR solution FB # 28 UV Blue
349 nm 430 nm
Unbleached Cotton
Invisible in solution. High photochemical decay rate. Often high background fluorescence. May be present due to household detergents
Solophenyl Direct Yellow 96 UV Yellow
397 nm 490 nm (pH sens.)
Unbleached Cotton
Sensitive to pH. No coloring of water. Usually low background
Sodium Fluorescein Acid Yellow 73 Yellow-Green
491 nm 512 nm
Activated Carbon 6–14 mesh
High photochemical decay and sensitive to pH. Probably the most frequently used tracer for karst studies. Low sorption
Eosin Acid Red 87 Red
516 nm 538 nm
Activated Carbon 6–14 mesh
High photochemical decay. Can overlap with sodium fluorescein if both are present in the same sample
Rhodamine WT CI Acid Red 388 Red
554 nm 580 nm
Activated Carbon 6–14 mesh
Low background. Good stability in sunlight. Moderate sorption. Generally requires a fluorometer for detection. Some environmental concerns
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is not called out at great expense. A few states do not allow any water tracing tests. Care should be taken to avoid coloration at water intakes. All work in a given area should have one coordinator so various tests cannot cross tracers. Some epikarst aquifers may take several years to clear a given dye from an injection point, so careful planning is needed to avoid confounding future tests using the same tracer. The following description of the basic procedures for conducting tracer tests in karst area is not intended to serve as a training manual. Tracers, like antibiotics, will lose their usefulness if overused or improperly used through time. The design (and cost) of a tracer test is guided by the questions the study is attempting to answer. Simple point to point tests such as determining the resurgence for a sinking stream are usually done qualitatively using passive detectors. Qualitative tests are often done for recognizance in previously unstudied areas and may then be used to design future quantitative tests. Studies to determine hydraulic properties of the karst flow system require quantitative tests. Some tracer tests are designed to predict possible contaminant transport and travel times for the movement of contaminants. Dissolved tracers may mimic the behavior of dissolved contaminants with similar retardation (sorptive) characteristics but some caution is needed in predicting the travel times, especially under different flow conditions or for particulates or NAPLs. A book on karst groundwater contamination (White et al., 2018) provides detailed discussions of this complicated topic.
Qualitative tracing using passive detectors Passive collectors such as packets of activated carbon or unbleached cotton are placed in the monitoring points and collected at regular intervals. The dye, if present, is then eluted from the carbon or exposed by examining the cotton under ultraviolet light. The use of a fluorometer to analyze the elutant increases the minimum detectable concentration of the tracer by several orders of magnitude. It is not possible to reliably obtain the dye concentrations at the resurgences from this technique, because exposure time of the detector to the dye and other variables affect the elutant dye concentration. Time of travel for the tracer can only be approximated based on the changing interval for the detectors. Detailed instructions for conducting qualitative tracer tests are presented in Jones (1984), Mull et al. (1988), Alexander Jr. and Quinlan (1996), and Taylor and Greene (2008). The principal groups of passive detectors and tracers are. 1. Activated carbon (activated charcoal) a. Sodium fluorescein b. Eosin c. Rhodamine WT, Sulforhodamine B 2. Cotton a. Tinopal CBS-X (FB 351) b. Phorwite c. Calcofluor white (FB number 28) d. Direct yellow 96 3. Plankton netting (about 25-μm mesh openings) a. Colored spores of L. calvatum The fluorescent dyes have been used extensively in North America and Europe while tracing using spores has been primarily a European technique. The following discussion outlines the basic procedure for conducting a qualitative tracer test using sodium fluorescein and carbon detectors. The execution of a tracer test should involve an initial survey of the study area to identify the hydrologic boundaries and all possible springs and resurgences. Any previous tracer tests from the area and all cave survey data should be studied. All land owners potentially affected by the test should be contacted and access secured to injection and monitoring points. Local and state agencies should be notified about the study and any required permits obtained. Measurements of background fluorescence from the springs should be used to help determine the most suitable tracers. Initial tests in an area should be conducted during average flow conditions; tests under minimal discharge should generally be avoided. The amount of dye to use for any given test is an educated guess at best. The quantity of tracer to be injected is a function the hydrologic conditions of the study area, the characteristics of the tracer, and the analytical procedure. Sodium fluorescein and the rhodamine dyes have a minimum detection concentration of around 0.006 μg/L (using a fluorometer). In practice, the detection limit will be a function of fluctuations in ambient natural fluorescence (background) and the predetermined increase in relative fluorescent intensity in the appropriate wavelength as defined in the laboratory protocol (typically 3–10 times background fluorescence) for the test to be considered positive. It is usually desirable to keep the dye recovery concentrations below the visible threshold (about 30 μg/L). It is best to try and use as much dye as possible and aim for recovery concentrations at the resurgences of between 0.1 and 100 μg/L and possibly higher if no water supplies could be affected. The possible travel distances and discharges of the resurgences may be used to help estimate the travel time and tracer quantity required for the test. Several formulae to estimate dye quantities are presented in K€ass (1998), but these provide only rough guidelines. Tests in conduit aquifers involving
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distances less than 1 km and discharges less than 30 L/s should require less than 200 g of sodium fluorescein. Most of the other commonly used tracer dyes probably require 2–4 times the mass used for a sodium fluorescein test. Less dye is usually needed for qualitative tests than for quantitative tests. If the possible coloration of the resurgences is not objectionable, the estimated amount of dye should be increased. Care must be taken to keep maximum tracer recovery concentrations below the visible threshold (about 30 μg/L for sodium fluorescein) if the tracer could affect a water supply. The US Geological Survey suggested the maximum dye concentration of 10 μg/L at water intakes, but this is a conservative number and is not based on toxicological considerations (Alexander and Quinlan, 1996). The monitoring locations should include all possible resurgences and one or two “impossible” ones to serve as controls for the test. Carbon detectors are usually made of envelopes of plastic screening material containing a few grams of activated carbon granules (about 6–14 mesh). Cotton detectors are usually balls of unbleached cotton. The passive detectors should be changed at regular intervals, usually between 2 and 14 days. The detectors should be in place for at least one changing interval prior to dye injection to establish background information. They should be placed to remain submerged with changing water levels, be readily recoverable, and remain out of sight to causal observers. An interesting “hanger” for detectors in deeper water was developed by James Quinlan and is shown in Fig. 2. The detectors should be placed so medium velocity current flows through them and they are not exposed to direct sunlight. Detectors lose their ability to adsorb the tracers through time, but they will retain the dye for periods of at least 1 year following exposure when kept in total darkness. Caution must be used to prevent contamination of the detectors and they should be placed before the dye is injected. Injection points for the tracer are ideally flowing streams at surface sink-points or upstream of impassible reaches in a cave. Powered dyes are often mixed with water prior to injection to avoid unwanted spread of the dye. The injection points should be chosen to minimize the surface exposure of the tracers to sunlight. Tests involving injection points without rapid continuous flow such as a dry sinkhole or water well are much more difficult and more prone to failure. Tracer tests from sinkholes or wells are usually only conducted by professional hydrologists. The basic procedure is to use a tank truck to deliver water to the site. Several thousand liters of water are run into the sinkhole before the dye is added. The dye is then flushed into the aquifer system with about an additional 10,000 L of water. Injection sites that use water wells often provide unsuccessful tests, which mean that the dye injected as the tracer will not be usable in the area for a number of years. Caution and careful planning are needed for this type of test. Another dye injection involves leaving the dye in a safe storage area in a presently dry sink or streambed and letting the next storm wash the tracer into the flow system. This “dry set” system works best in remote areas where people or animals are unlikely to encounter the tracer before it is removed by precipitation. The analysis of the carbon for the presence of sodium fluorescein involves the following steps: 1. Rinse the carbon in clean water to remove sediment 2. Fill a test tube about half full with the carbon
FIG. 2 A “Quinlan Gumdrop” anchor for passive dye collectors. The base is concrete molded in a coffee can and the detectors are suspended above the bottom of deeper springs or streams. The screen envelops contain unbleached cotton (right arm) and activated carbon (left arm and bottom). The packets can also be secured with wire to rocks or other solid anchors such as old railroad spikes.
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3. Pour in the test solution, called the eluent, to cover the carbon about 3 cm deep (5–10 mL) 4. Let the samples sit undisturbed in the dark for between 30 min and 24 h 5. Shine a concentrated white light source such as a focusable flashlight through the test tube and look for the characteristic fluorescent green sheen on top the carbon granules (do not shake or disturb the solution for a visual examination) Most of the tests conducted today use a fluorometer to analysis the elutant rather than relying on visual examination as described above. Some workers prefer to use a larger diameter vial to increase the length of the light path through the elutant. The most commonly used eluent for sodium fluorescein is 5% potassium hydroxide (KOH) in 70% isopropyl alcohol. Many other eluents have been used including the “Smart Solution” (50% 1-propanol, 20% NH4OH, and 30% distilled water by volume). It takes some practice to distinguish weakly positive sodium fluorescein tests from some naturally occurring background. Numerous trials at varying dye concentrations should be run under controlled conditions before moving on to actual field tests. One of the advantages of the passive collector system is that the samples are time integrated for the exposure period of the detectors (Fig. 3). The amount of dye adsorbed and subsequently eluted from the carbon detectors is a function not only of the concentration of the dye passing the sampling site but the exposure time to the carbon, the surface area of the carbon in contact with the dye and the flow rate through the detector package. The lab protocol, especially the time allowed for the elution process, also affects the final apparent dye recovery concentration from the carbon. Some researchers (Field, 2002) object to the nonquantifiable nature of tests conducted using passive detectors for monitoring but the advantages in terms of being able to detect dyes at recovery concentrations that would be below the detection limits for water samples and the cost efficiency make the used of carbon detectors a primary tool for tracer tests (Aley, 2016).
Wishing well to emory spring 25
Water (ppb) Carbon (ppb)
Rhodamine WT (ppb)
20
15
10
5
0 1
2
3
4
5
6
7
Time (days) FIG. 3 Bar graph illustrating the dye concentration integrating effect from carbon detectors compared to discrete water samples. Carbon detectors were changed at 24-h intervals and water samples were collected at the time the carbon detectors changed.
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Quantitative water tracing in karst areas Many of the professionally conducted water tracing tests are quantitative. Resurgences are sampled at short discrete time intervals and the water is analyzed using a calibrated fluorometer to determine the concentration of the dye in each sample. Several different dyes may be injected simultaneously so multiple sink points may be tested at the same time and under the same flow conditions. The results are plotted on a graph showing the concentration of the dye through time (breakthrough curve). The expense and complexity of quantitative tests are much higher, but considerably more information is obtained about the nature of the subsurface flow systems. The combination of quantitative dye tracing results with discharge measurements allows a more certain delineation of karst drainage basins and the calculation of the percentage of discharge from different sources at a given resurgence. Good results are dependent on using an appropriate tracer, adequate sampling frequency, and accurate analysis of the samples. A paper by Field (2003) provides formulas for determining dye mass and sampling frequency, but a good estimation of hydraulic and transport characteristics of the system is needed. This usually requires advance knowledge of the flow conditions and the resurgence(s). An estimate of the expected travel time from sink to resurgence is also helpful in planning tests. Flow velocities for conduit karst aquifers are typically in the range of 0.2–7 km/day, but some tracer tests have shown results far outside of these values. The planning and dye injection procedure is essentially the same as described for qualitative traces. Qualitative tests are often conducted first to identify which resurgences to monitor and to estimate the travel time for the tracer. Most tracer tests in karst areas use an instantaneous injection of the dye, but continuous (over several hours) releases are occasionally used to facilitate the calculation of discharge at the recovery point. Fluctuations in background fluorescence are often a problem in the blue and green wavelengths, so the red dye Rhodamine WT is the most commonly used fluorescent dye for quantitative studies. Quantitative tests generally require about twice as much dye as qualitative tests. The much greater sampling effort required for quantitative testing is the major reason for the increase in cost. The instrument used to determine fluorescent dye concentrations is a fluorometer (Fig. 4). Fluorometers pass light through the sample at a controlled excitation wavelength and measure the relative fluorescent intensity of the output signal at the emission wavelength. Laboratory standards at different dye concentrations must be prepared for each batch of dye to calibrate the fluorometer to determine dye concentrations. Filter fluorometers may be laboratory or field instruments and use various filters to set the excitation and emission wavelengths, while scanning spectrofluorophotometers are laboratory instruments that are better suited to analyzing samples containing multiple dyes or having high fluorescent backgrounds. A continuous scan through the wavelengths of interest with about a 20-nm separation between the excitation and emission settings allows a single scan to be analyzed for several different dyes. Some workers use computer enhancement of the instrument output signal to help identify and separate overlapping fluorescent peaks. Spectrofluorophotometers are more selective in their ability to isolate specific fluorescent compounds, but both types of fluorometers are highly sensitive to low concentrations of dye. Recording filter fluorometers provide a high-resolution record of the passage of the dye pulse, but spectrofluorophotometers are more reliable in the identification and quantification of individual tracers. Filter fluorometers may be fitted with a flow-through door which circulates water past the instrument’s light source. This provides a continuous record of fluorescent intensity at one set of wavelengths, but this is usually only practical for tests with travel times of less than 24 h. Fluorometric probes compatible with data loggers make continuous fluorometric measurements more cost-effective for many field studies and record a nearly continuous picture of the dye pulse-through time. Handheld
FIG. 4 A scanning spectrofluorometer used for laboratory quantification and identification of fluorescent dyes and a battery powered hand-held fluorometers (lower left) used for field monitoring of a dye pulse.
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fluorometers with several “channels” set for specific tracer dyes are robust enough to take into caves, but the samples must be taken manually. Most quantitative tracer studies at present involve the use of automatic water samplers at several resurgences over a period of days to weeks. The sampling interval is dependent on the expected travel time of the tracer and may range from 15 min to 1 per day. Some resolution in the shape of the dye recovery curve may be lost if the sampling interval is too long. Sampling should continue for a time sufficient to ensure that most of the dye pulse has been recovered. Samples should be stored in the dark in glass containers to minimize photochemical decay and sorption of the dye to the sample bottles.
Interpretation of water tracing tests Qualitative tracer tests are positive if the dye is detected at one or more monitoring points. A successful tracer test may prove a hydrologic connection between the injection point and recovery point(s) of the tracer, but the exact flow path(s) will remain unknown. Depending on the changing interval for the detectors, some information of the travel time for the tracer may be available. If the tracer is not detected at a monitoring station, it is listed as “none detected” rather than “negative.” Negative tests do not prove that a connection does not exist and the percentage of the injected tracer actually recovered can only be determined by quantitative measurements of the dye concentration and discharge at the resurgences during the recovery period of the dye pulse. Tracer tests are representative of the karst flow system for the hydrologic conditions prevailing at the time of the test. Overflow routes may exist that function only during high-flow conditions, so a thorough study of a karst region will require multiple tests at different water levels. Some resurgences may only function during high-flow conditions. The travel times for the tracers will be less at high water levels, and the dye will move much faster if the conduits are completely flooded. Quantitative tests between the same injection and recovery sites under a range of flow conditions may be used to develop the relationship between travel time and groundwater discharge and to predict solute transport characteristics. Discharge-dependent thresholds may be identified. The graph of dye recovery concentration plotted through time is called a dye recovery or breakthrough curve (Fig. 5). This curve presents a picture of the passage of the dye “cloud” or pulse-through time at the stationary sampling point. Breakthrough curves are typically skewed to the right with rapid rise to peak (maximum) dye concentration and a long trailing edge of declining dye concentrations. The shape or form of the breakthrough curve is a function of the characteristics of the tracer, the flow conditions during the test, and the characteristics of the aquifer or conduit system. The mean flow velocity is usually computed using the straight-line distance between sink and resurgence and the time between dye injection and recovery of half of the tracer. This is the “centroid” or center of recovered mass on the breakthrough curve. Some workers use the time to peak concentration if the resurgences are not completely sampled through the long trailing period of declining tracer concentrations. If discharge is known for throughout the dye recovery period, the amount of dye recovered can be calculated by integrating the area under the breakthrough curve and multiply by the discharge: 25
Fluorescein sodium (PPB)
Dye recovery at organ cave spring Greenbrier County, West Virgina 20
15
10
5
0 40
60
80
100
120
Hours from dye injection FIG. 5 Graph showing a simple breakthrough curve from a tracer test from a cave to a spring. The principal components of the curve are (1) first arrival of the dye at 30 h; (2) peak (maximum) recovery concentration at 42 h; (3) dye centroid (center of mass and median recovery point) at 50 h; and (4) total time of dye cloud passage from 30 through 120 + hours. Note the 4-h sampling interval and that sampling was stopped before the complete passage of the dye cloud.
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FIG. 6 Graph showing dye recovery (red line) and the percent recovery of injected dye (dashed blue) line through time. Sampling was terminated before the passage of the dye cloud was complete but it is probable that this spring accounts for all the water from the tracer injection site.
80
7
70
6
60
5
50
4
40
3
30
2
20
1
10
0
Percent recovery
FL(PPB)
Culverson creek august 2009 8
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0 8
10
12
14
16
18
20
22
Time (days)
M¼Q
ð∞ Cdt 0
where M ¼ mass of dye recovered. Q ¼ discharge. C ¼ dye concentration at time t. Discharge is usually based on current-meter measurements during the test. If the tracer is conservative (not subject to decay or sorptive losses) and all of the water from the injection point is accounted for, the mass of dye recovered should be equal to the mass injected. This is the dye budget for the test (Fig. 6). None of the fluorescent tracers are completely conservative, but tracer tests with travel times of less than a few days should balance reasonably well if all of the resurgences have been monitored. Assuming complete mixing and a conservative tracer, the area under the time-concentration breakthrough curve should be constant even as the shape of the curve changes in the downstream direction. A significant loss of the tracer from the system suggests that additional resurgences are present or some of the tracer has moved into storage areas along the flow route. Less conservative tracers usually produce a longer travel time estimate due to retardation caused by sorption and desorption along the flow route. Breakthrough curves for many tracer tests in karst areas exhibit multiple peaks, and the time between the peaks may be dependent on discharge. The interpretation of these tests is somewhat subjective, but much insight into the internal flow characteristics of the karst aquifer and the conduit system can be gained from these tests. The best breakthrough curve data are obtained using continuous flow fluorometry during the passage of the dye pulse. The shape or form of the dye cloud is dependent on the interaction of dispersion, dilution, divergence and convergence, and storage (Smart, 1988). The quantitative interpretation of breakthrough curves is generally based on solute transport equations. An introduction to this type of analysis was presented in Benischke et al. (2007) and a computer program to analyze tracer recovery data was developed by Field (2002). Traces involving longer distances and longer travel times show lower peak concentrations and longer persistence of the dye pulse as the tracer becomes increasingly more dispersed with time and distance. The shape of the tracer cloud becomes more asymmetrical as dispersion is influenced by the tracer moving in and out of pockets of “dead zone storage” along the flow route. Dispersion is also affected by the retardation factor for tracers that are less than completely conservative. Divergence occurs where the underground flow routes divide or break away from the main conduit. Convergence is where the routes rejoin the main conduits. The divergent routes generally are longer flow paths than the main conduit, so the portion of the flow and tracer diverted may reappear as second or multiple pulses at the sampling station. Dilution is caused by unlabeled tributary water mixing with the water containing the original tracer injection. This will cause a lower concentration of dye in the downstream water samples. Storage of the tracer in the conduit system may be very complex. Storage may just represent very low-velocity flow-through one section of a branching conduit, called in-line storage. A test done under conditions of diminishing flow may leave some of the
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High flow SPR 1
2
1
Time
Conc
Distribution 2
Time
1
Conc
2
1
Conc
Storage
SPR 1 Time
2
SPR 2
Time
Time
Conc
Anastomosing channels
1
Conc
1
SPR 2
Time
Conc
Conduits
SPR 2
Conc
Springs
Conc
Dye injection
Dilution
Low flow
Time
Time
FIG. 7 Sketches showing conceptual models of possible dye breakthrough curves. The pattern of the recovery curve is a function of prevailing water levels at the time of the test and the geometry of the conduit system. (From W.K. Jones, W. K., 1984. Analysis and interpretation of data from tracer tests in karst areas. Natl. Speleol. Soc. Bull. 46 (2), 41–47.; after Smart, C. C., Ford, D. C., 1982. Quantitative dye tracing in a glacierized alpine karst. Beitr. Geol. Schweiz-Hyrdrol., 28(1), 191–200. With permission.)
dye abandoned in upper level pockets to be remobilized when the flow increases at a later date. Some dye may become stored offline in lateral areas along the route such as small voids or in pores in the sediments. In-line storage tends to create dye pulses that correlate with flushing due to storm events. Dye from off-line storage tends to drain slowly during groundwater recessions. Some conceptual models of breakthrough curves for different flow levels and conduit geometries are presented in Fig. 7 (adapted from Smart and Ford, 1982). The best picture of the conduit aquifer system for any karst area is from the actual surveying of the cave passages. Tracer tests, interpreted in the context of the regional geology and hydrologic setting, can be used to fill in the blank place on the map where actual exploration of the caves is not possible. The rather unusual mixture of quantitative techniques and subjective interpretation of the unseen part of the aquifer makes water tracing a fascinating endeavor.
See also the following articles Hydrogeology of Karst Aquifers
Bibliography Alexander Jr., E.C., Quinlan, J.F., 1996. Practical tracing of groundwater with emphasis on karst terranes. In: Guidelines for Wellhead and Springhead Protection Area Delineation in Carbonate Rocks (prepared by Eckenfelder Inc.), pp. 1–38. EPA 904-B-97-003, Appendix B. Aley, T., 2016. Using activated carbon samplers to improve detection of fluorescent tracer dyes in groundwater remediation studies. In: Rectanus, H.V., Rodgers, P. (Eds.), Remediation of Chlorinated and Recalcitrant Compounds. Tenth International Conference on Remediation of Chlorinated and Recalcitrant Compounds, A-056. www.battelle.org/chlorcon. Behrens, H., Beims, U., Dieter, H., Dietze, G., Eikmann, T., Grummt, T., Hanisch, H., Henseling, H., Kass, W., Kerndorff, H., Leibundgut, C., Muller-Wegener, U., Ronnefahrt, I., Scharenberg, B., Scharenberg, B., Schleyer, R., Schloz, W., Tilkes, F., 2001. Toxicological and ecotoxicological assessment of water tracers. Hydrogeol. J. 9, 321–325.
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Benischke, R., Goldscheider, N., Smart, C., 2007. Tracer techniques. In: Goldscheider, N., Drew, D. (Eds.), Methods in Karst Hydrogeology. Taylor and Francis/ Balkema, pp. 147–170. Field, M.S., 2002. The QTRACER2 Program for Tracer-Breakthrough Curve Analysis for Tracer Tests in Karstic Aquifers and Other Hydrologic Systems. U.S. Environmental Protection Agency. 600/R-02/001. 179 p. Field, M.S., 2003. A review of some tracer-test design equations for tracer-mass estimation and sample collection frequency. Environ. Geol. 43, 867–881. Ford, D.C., Williams, P.W., 2007. Karst Hydrogeology and Geomorphology. p. 195, Wiley, West Sussex. Freixes, A., Monterde, M., Ramoneda, J., 1997. Tracer tests in the Joeu karstic system (Aran Valley, Central Pyrenees, NE Spain). In: Kranjc, A. (Ed.), Tracer Hydrology 97. A. A. Balkema, Rotterdam, The Netherlands, pp. 219–225. Gabrovsek, F., Kogovsek, J., Kovacic, G., Petric, M., Ravbar, N., Turk, J., 2009. Recent results of tracer tests in the catchment of the Unica River (SW Slovenia). Acta Cardiol. 39 (1), 27–37. Goldscheider, N., Meiman, J., Pronk, M., Smart, C., 2008. Tracer tests in karst hydrogeology and speleology. Int. J. Speleol. 37 (1), 27–40. Jones, W.K., 1984. Analysis and interpretation of data from tracer tests in karst areas. Natl. Speleol. Soc. Bull. 46 (2), 41–47. K€ass, W., 1998. Tracing Technique in Geohydrology. A. A. Balkema, Rotterdam, The Netherlands. 581 pp. Mull, D.S., Liebermann, T.D., Smoot, J.L., Woosley Jr., L.H., 1988. Application of dye-tracing techniques for determining solute-transport characteristics of ground water in karst terranes. In: U.S. Environmental Protection Agency, EPA. p. 103 904/6-88-001. Smart, P.L., 1984. A review of the toxicity of twelve fluorescent dyes used for water tracing. Natl. Speolol. Soc. Bull. 46 (2), 21–33. Smart, C.C., 1988. Artificial tracer techniques for the determination of the structure of conduit aquifers. Ground Water 26 (4), 445–453. Smart, C.C., 2005. Error and technique in fluorescent dye tracing. In: Beck, B.F. (Ed.), Sinkholes and the Engineering and Environmental Impacts of Karst. American Society of Civil Engineers, Geotechnical Special Publication 144, pp. 181–200. Smart, C.C., Ford, D.C., 1982. Quantitative dye tracing in a glacierized alpine karst. Beitr. Geol. Schweiz-Hyrdrol. 28 (1), 191–200. Smart, P.L., Laidlaw, I.M.S., 1977. An evaluation of some fluorescent dyes for water tracers. Water Resour. Res. 13, 15–33. Taylor, C.J., Greene, E.A., 2008. Hydrogeologic characterization and methods used in the investigation of karst hydrology. In: Rosenberry, D.O., LaBaugh, J.W. (Eds.), Field Techniques for Estimating Water Fluxes Between Surface Water and Ground Water, pp. 75–144. Chapter 3. White, W.B., Herman, J.S., Herman, E.K., Rutigliano, M., 2018. Karst Groundwater Contamination and public health. Advances in Karst Science, Springer. 347pp. Wilson, J.F., 1968. Fluorometric Procedures for Dye Tracing. Techniques of Water-Resources Investigations of the U. S. Geological Survey, Book 3. Chapter A12 (Revised). pp. 1–34.
Water tracing in karst aquifers William K. Jones Karst Waters Institute, Leesburg, VA, United States
Definitions and objectives of tracer tests in karst Water tracer tests are usually conducted to establish the hydrologic connections between two or more points. The tracer is an identifiable label or marker added to flowing water that establishes the links between the injection point of the tracer and the monitoring points where the tracer reappears. Fluorescent dyes are the most commonly used tracers in karst aquifers, but a wide range of substances has been used successfully. The experimental design of a tracer test may be qualitative to simply establish if a hydrologic connection exists between two points, or quantitative to measure the time-concentration series (breakthrough curve) generated by the recovery of the tracer. Water tracer tests usually work well in karst areas because of the fast groundwater flow rates and the prevalence of flow paths restricted to discrete conduits. The level of effort and the cost of conducting a tracer test is a function of the question being asked. Many tracer tests are an extension of cave exploration and are conducted to “see where the water goes.” The tracer test may have the relatively simple objective of establishing a connection between two cave passages separated by an impassable reach. Tracer tests may be used to determine the destination of water flowing into a sinkhole or blind valley. The karst drainage basin contributing water to a spring or resurgence may be estimated based on the results of tracer experiments. The interbasin transfer of water is frequently demonstrated by tracer tests. The travel rate of a tracer may be used to establish some of the hydrologic properties of an aquifer. Examination of the breakthrough curve of a tracer test can aid in the interpretations of some of the internal flow characteristics of the aquifer. The movement of a tracer may mimic the subsurface behavior of a pollutant introduced into the aquifer at the same point. Tracers may be used to identify the source of pollution at a spring. Tracer tests are sometimes used to demonstrate the “vulnerability” of karst aquifers to chemical spills and pollution associated with inappropriate land use practices and to establish “wellhead protection areas” for public water supplies. A study of the Unica River basin in Slovenia (Gabrovsek et al., 2009) provided a demonstration of the range of hydrologic information that can be obtained from tracer tests and the applicability to the protection and management of karst water resources. Naturally occurring chemicals or isotopes may be used to determine the age or residence time of water at sampling points within the aquifer. The sources of the water in terms of percentages contributed by conduit flow versus drainage from the overlying epikarst or deeper fracture zones may be studied. These chemical markers are considered tracers, but they identify the various components or storage areas of the aquifer rather than establish direct hydrologic connections or paths between specific points in the aquifer.
History of water tracing in karst aquifers A considerable body of folklore concerning underground water connections has accumulated throughout the world’s cave areas. Tests have been reported using tracers such as wheat chaff, duck feathers, marked logs, tagged eels, and muddy water from storm events. A 2000-year-old tracer test is attributed to tetrarch Philippus who supposedly established the source of the springhead of the Jordan River in 10 AD. Chaff was thrown into Phialo Pond (Berekhat Ram Crater Lake) and reemerged from the cave spring Panium, the head of the Jordan River. Although the results of this test have been questioned, it does establish that the basic idea of tracing underground water is quite old. A scientifically planned tracer test was conducted in 1872 by A. Hagler to determine the origin of a typhoid fever outbreak in the village of Lausen in Switzerland. About 800 kg of salt (NaCl) was injected in a sinking stream on a farm south of the village and the water supply spring for the village showed a strong reaction for chloride the following day (K€ass, 1998). The start of modern water tracing studies followed the discovery of fluorescein by A. V. Baeyer in 1871. The sodium salt of fluorescein, called uranine in Europe, rapidly became the most used and probably the most successful groundwater tracer for cave 1144
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FIG. 1 Fluorescein sodium powder is red but turns bright fluorescent green when dissolved in water. Here the dye is being added to an underground stream just upstream of an impassible sump. (Photo by Philip C. Lucas. Used with permission.)
and karst studies to this day (Fig. 1). The first reported use of sodium fluorescein as a tracer was an experiment conducted in southwestern Germany on October 9, 1877, by Professor Albert Knop. Ten kilograms of sodium fluorescein supplied by C. Ten Brink was injected at infiltration points in the bed of the Danube River and reappeared 2 days later and 12 km away at the Aach Spring, a tributary of the Rhine River. A test conducted a couple of weeks earlier from the same location used almost 10,000 kg of salt and may have been the first quantitative tracer test with hourly sampling at the Aach Spring. Work on the development of methods for tracing underground waters continued in Europe. Many of the early tracer tests used large quantities of dye, because detection depended on visual coloring of the water at the resurgence. An interesting example of an accidental tracer experiment occurred in the French Franche-Comte in 1901. As the result of a fire at the Pernod distillery at Pontarlier, a quantity of absinthe poured into the Doubs River. A significant amount of the alcohol seeped through the bed of the river and reemerged the following day 10 km away at the source spring of the Loue River. The test of the connection between the Doubs and the Loue was repeated by E. A. Martel in 1910 using 100 kg of sodium fluorescein. The Loue was brightly colored 2 days later for a distance of 100 km downstream. The source of the Garonne River in southern France was determined by a tracer test in 1931 by Norbert Casteret. He injected 60 kg of sodium fluorescein at the Trou du Toro on the Spanish side of the Pyrenees and it emerged 3.7 km away and 10h later at the Goueil de Joueou on the French side. The Garonne River was colored a bright green for over 50 km downstream. Quantitative tests at the same site in the early 1990s used less than 2 kg of sodium fluorescein and yielded travel times for the tracers of about 11 h during high-flow conditions in the summer and 180 h for a low-flow test in the winter (Freixes et al., 1997). Various tracer techniques were tried in Europe through the 1920s. A “fluoroscope” was developed which allowed a rough estimation of dye concentrations in water samples by visual comparison with prepared laboratory standards. A number of different salts, dyes, and even radioactive compounds were tried. Bacteria were tried as a particulate drift tracer as early as 1896. Spores from the club moss Lycopodium calvatum were suggested as a possible tracer in 1910, and the first reported tracer test using spores is from Europe in 1940 (K€ass, 1998). Spores became a popular tracer in Europe in the 1950s because they could be dyed different colors and several sink-points could be traced simultaneously. Plankton nets were suspended at the springs to passively collect the spores, so constant surveillance of a number of springs was no longer required. Spores dyed with fluorescent dyes proved to be easier to identify under a microscope. Drifting tracers such as spores and bacteria are still used to a certain extent in Europe but have never been popular in North America. One novel tracing experiment in Slovenia in 1929 used marked eels. The dorsal fins of 494 eels were notched and the eels were released in the Reka River that sinks into Skocjanske Cave and resurges 34 km to the west at the Timavo Springs in Trieste Bay (Italy). Twenty-nine of the eels were caught in eel pots at the Timavo Springs during the 1-year observation period. The Yorkshire Geological Society carried out a series of tracer tests in the Ingleborough area of Great Britain in 1904. One principle from this work has been to test “key sinks” and to guess the remainder in lieu of testing everything. This approach is still practiced in many karst areas, but more intensive studies at later dates sometimes produce surprising results. Much of the subsequent work on tracing underground drainage in the British Isles has been conducted by caving groups associated with various universities. A study of the caves of northwest County Clare, Ireland, in the early 1960s by E. K. Tratman and the University of Bristol Speleological Society, involved a number of tracer tests using sodium fluorescein and passive carbon detectors to monitor the resurgences. Just as professional cave and karst studies in North America lagged behind Europe in general, so did the interest in tracing underground water. The first description of tracer tests from North America was a U. S. Geological Survey water supply paper by
1146 Encyclopedia of Caves
R. B. Dole in 1906. This paper discussed the use of sodium fluorescein as a tracer, but few tracer tests are reported from North America before the late 1950s. Whereas most of the water tracing studies in Europe were conducted by professional hydrologists or speleologists, often at considerable expense, the real beginning of karst water tracing studies in North America was the contribution of unpaid cavers as an extension of cave exploration. A brief description of a method to recover sodium fluorescein sorbed onto activated coconut charcoal granules was published by J. R. Dunn in a cave club newsletter in 1957. This discovery offered an inexpensive technique to monitor different resurgences without the necessity of constant surveillance. The researchers could place carbon packets (called “Dunn Bugs”) in the springs, inject the dye in a sinking stream or cave, and return in a few weeks to collect and test the carbon packets. The concentration of the dye on the carbon increases with the exposure time to the dye, so recovery concentrations of sodium fluorescein below the normal visual threshold could be detected. The testing relied on visual identification of the dye as a fluorescent yellow-green sheen floating on top of the carbon in an elutant. Apparently a very similar technique was described by Mayrhofer in 1904 (K€ass, 1998), but the activated carbon system was used in Europe more as a technique for enriching the dye concentration in a sample than as an unattended monitoring system. The first field test of the passive detector system in North America was described by T. D. Turner in 1958. Turner reported a tracer test using carbon detectors and 4 g of sodium fluorescein over a distance of 4.8 km near Pine Grove Mills, PA. Water tracing in North American cave studies really began in the early 1960s using sodium fluorescein and passive detectors of activated carbon. Hermine Zotter began a dye tracer study in Pocahontas County in east central West Virginia in 1960. Thomas Aley conducted a tracer test for a court case in California in 1963. This was probably the first use of underground water tracing to resolve litigation in the United States. Aley worked on a study for the US Forest Service in the Ozarks in Missouri in 1966 and conducted a 64-km long trace from the Eleven Point River Basin to Big Spring on the Current River Basin. William Jones conducted a study for the U.S. Geological Survey in Greenbrier County, West Virginia, in 1966. The results of the tracing tests were coupled with the positions of surveyed cave streams and topographic divides to delineate karst drainage basins. Charles Brown was conducting tracer tests in the Maligne basin in Alberta, Canada, by 1969. James Quinlan became the park geologist for Mammoth Cave National Park in Kentucky in 1973 and started an intensive series of water tracing tests. The results of the tracing tests were combined with data from cave surveys and potentiometric contours to produce a detailed map of drainage basins and subbasins around Mammoth Cave (Ford and Williams, 2007). The development in the early 1960s of fluorometers that control or measure light emissions in both the excitation and emission wavelengths of different fluorescent compounds enabled dye analytical methods to become much more quantitative. Quantitative water tracing using fluorometers dates from the early 1960s. Much of the early quantitative tracer work was for surface water timeof-travel studies. Fluorometers also allowed several different fluorescent compounds to be used simultaneously because the instruments could be set to narrow specific wavelengths. Much of the development of quantitative tracer techniques was presented in a series of conferences in Europe beginning with the Specialists Conference on Tracing of Subterranean Waters in Graz, Austria, in 1966. This conference series continued at about 4-year intervals as the Symposium on Underground Water Tracing (SUWT) conferences. The published proceedings from the series contain a wealth of information on various tracer techniques for both karst and porous media water tracing. Much of the material from these conferences is distilled in the book by K€ass (1998). A U. S. Geological Survey paper by Wilson (1968) and a paper on the evaluation of fluorescent tracers by Smart and Laidlaw (1977) were probably the most influential papers on quantitative water tracing published in North America.
Tracers Groundwater tracers are generally classified as: (1) naturally occurring (often accidental in their addition to the water), (2) artificial tracers deliberately introduced into the aquifer, and (3) pulses. Naturally occurring tracers include the chemical constituents and isotopes naturally found in water or present due to the activities of man. Tritium, fluorocarbons, pollutants from waste sites, and thermal waters may all serve as tracers in some circumstances. Isolated thunderstorms may generate a distinctive flow response at springs some distance from the storm. Organisms unique to a particular subsurface drainage basin may be considered a tracer if their distribution can be used to define the boundaries of the basin. Naturally occurring tracers are often used to “date” the water, artificial tracers are used to label a specific sink or cave, and pulses send an identifiable signal through the conduit or sometimes a large part of the aquifer. Most tracer tests in karst areas are conducted using artificially injected tracers. These tracers may be broadly classified as: (1) water-soluble tracers (dyes, salts), (2) particulate or drift material tracers (club moss spores), and (3) physical pulses (flood waves). The ideal tracer should be: 1. 2. 3. 4.
Nontoxic Not normally present in the study area (low background) Detectable at very low concentrations Conservative (minimal sorptive and decay losses)
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5. 6. 7. 8.
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Have the same density as water (or be neutrally buoyant) Relatively inexpensive Easy to sample using passive collectors Easy to analyze and quantify in the laboratory
The vast majority of tracer tests in cave and karst areas are conducted using fluorescent dyes. All fluorescent dyes have some tendency to adsorb to clay minerals and some have high photochemical decay rates. The relative fluorescent intensity of many dyes is pH and temperature dependent. These considerations are usually not a problem for underground tracing in cavernous aquifers with high flow rates. Tracer tests in diffuse-flow aquifers often involve the use of more conservative isotopes where the longer residence times make fluorescent dyes less suitable. A list of the principal fluorescent tracer dyes commonly used in North American karst studies is presented in Table 1. Tracer tests from Europe have largely been conducted quantitatively and required direct sampling of the resurgences. Studies in the British Isles and North America through 1990 have mostly been qualitative and used passive collectors to monitor the resurgences. Qualitative tracing is still used for reconnaissance type studies and work in remote areas, but quantitative tracing techniques are now the norm for most scientific studies throughout the world. The fluorescent dyes commonly used as tracers are believed to be quite harmless to people and the environment. Reviews of the toxicity data for tracer dyes were presented by Smart (1984) and Behrens et al. (2001). Several dyes in the rhodamine group, especially Rhodamine B, are not recommended due to environmental concerns, but the high sorptive tendency for some of these dyes make them a poor choice for groundwater tracing under any circumstances. A lot of confusion exists over the names of many of the tracers. Sodium fluorescein is known as uranine in Europe and commonly called fluorescein in the North American literature. Only the sodium salt of fluorescein is soluble enough to make it an efficient tracer. When describing a dye, the Color Index name should be cited at least once. Sodium fluorescein has a Color Index constitution number CI 45350 and a generic name of acid yellow 73. Also, all dye shipments should be checked when they are received to make certain the correct product is being used. Note that a few words of caution are in order for anyone planning on conducting tracer tests. Specialists now do most dye tracing, and water tracing by untrained people may be very damaging. In the United States, some states require a permit for conducting a tracer test, and the local water inspectors should always be notified in advance of any test so a hazardous materials team
TABLE 1 Principal fluorescent tracers used in North American karst studies Tracer Colour index name Color
Excitation Emission Maximum
Passive Detector
Remarks
Phorwite AR solution FB # 28 UV Blue
349 nm 430 nm
Unbleached Cotton
Invisible in solution. High photochemical decay rate. Often high background fluorescence. May be present due to household detergents
Solophenyl Direct Yellow 96 UV Yellow
397 nm 490 nm (pH sens.)
Unbleached Cotton
Sensitive to pH. No coloring of water. Usually low background
Sodium Fluorescein Acid Yellow 73 Yellow-Green
491 nm 512 nm
Activated Carbon 6–14 mesh
High photochemical decay and sensitive to pH. Probably the most frequently used tracer for karst studies. Low sorption
Eosin Acid Red 87 Red
516 nm 538 nm
Activated Carbon 6–14 mesh
High photochemical decay. Can overlap with sodium fluorescein if both are present in the same sample
Rhodamine WT CI Acid Red 388 Red
554 nm 580 nm
Activated Carbon 6–14 mesh
Low background. Good stability in sunlight. Moderate sorption. Generally requires a fluorometer for detection. Some environmental concerns
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is not called out at great expense. A few states do not allow any water tracing tests. Care should be taken to avoid coloration at water intakes. All work in a given area should have one coordinator so various tests cannot cross tracers. Some epikarst aquifers may take several years to clear a given dye from an injection point, so careful planning is needed to avoid confounding future tests using the same tracer. The following description of the basic procedures for conducting tracer tests in karst area is not intended to serve as a training manual. Tracers, like antibiotics, will lose their usefulness if overused or improperly used through time. The design (and cost) of a tracer test is guided by the questions the study is attempting to answer. Simple point to point tests such as determining the resurgence for a sinking stream are usually done qualitatively using passive detectors. Qualitative tests are often done for recognizance in previously unstudied areas and may then be used to design future quantitative tests. Studies to determine hydraulic properties of the karst flow system require quantitative tests. Some tracer tests are designed to predict possible contaminant transport and travel times for the movement of contaminants. Dissolved tracers may mimic the behavior of dissolved contaminants with similar retardation (sorptive) characteristics but some caution is needed in predicting the travel times, especially under different flow conditions or for particulates or NAPLs. A book on karst groundwater contamination (White et al., 2018) provides detailed discussions of this complicated topic.
Qualitative tracing using passive detectors Passive collectors such as packets of activated carbon or unbleached cotton are placed in the monitoring points and collected at regular intervals. The dye, if present, is then eluted from the carbon or exposed by examining the cotton under ultraviolet light. The use of a fluorometer to analyze the elutant increases the minimum detectable concentration of the tracer by several orders of magnitude. It is not possible to reliably obtain the dye concentrations at the resurgences from this technique, because exposure time of the detector to the dye and other variables affect the elutant dye concentration. Time of travel for the tracer can only be approximated based on the changing interval for the detectors. Detailed instructions for conducting qualitative tracer tests are presented in Jones (1984), Mull et al. (1988), Alexander Jr. and Quinlan (1996), and Taylor and Greene (2008). The principal groups of passive detectors and tracers are. 1. Activated carbon (activated charcoal) a. Sodium fluorescein b. Eosin c. Rhodamine WT, Sulforhodamine B 2. Cotton a. Tinopal CBS-X (FB 351) b. Phorwite c. Calcofluor white (FB number 28) d. Direct yellow 96 3. Plankton netting (about 25-μm mesh openings) a. Colored spores of L. calvatum The fluorescent dyes have been used extensively in North America and Europe while tracing using spores has been primarily a European technique. The following discussion outlines the basic procedure for conducting a qualitative tracer test using sodium fluorescein and carbon detectors. The execution of a tracer test should involve an initial survey of the study area to identify the hydrologic boundaries and all possible springs and resurgences. Any previous tracer tests from the area and all cave survey data should be studied. All land owners potentially affected by the test should be contacted and access secured to injection and monitoring points. Local and state agencies should be notified about the study and any required permits obtained. Measurements of background fluorescence from the springs should be used to help determine the most suitable tracers. Initial tests in an area should be conducted during average flow conditions; tests under minimal discharge should generally be avoided. The amount of dye to use for any given test is an educated guess at best. The quantity of tracer to be injected is a function the hydrologic conditions of the study area, the characteristics of the tracer, and the analytical procedure. Sodium fluorescein and the rhodamine dyes have a minimum detection concentration of around 0.006 μg/L (using a fluorometer). In practice, the detection limit will be a function of fluctuations in ambient natural fluorescence (background) and the predetermined increase in relative fluorescent intensity in the appropriate wavelength as defined in the laboratory protocol (typically 3–10 times background fluorescence) for the test to be considered positive. It is usually desirable to keep the dye recovery concentrations below the visible threshold (about 30 μg/L). It is best to try and use as much dye as possible and aim for recovery concentrations at the resurgences of between 0.1 and 100 μg/L and possibly higher if no water supplies could be affected. The possible travel distances and discharges of the resurgences may be used to help estimate the travel time and tracer quantity required for the test. Several formulae to estimate dye quantities are presented in K€ass (1998), but these provide only rough guidelines. Tests in conduit aquifers involving
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distances less than 1 km and discharges less than 30 L/s should require less than 200 g of sodium fluorescein. Most of the other commonly used tracer dyes probably require 2–4 times the mass used for a sodium fluorescein test. Less dye is usually needed for qualitative tests than for quantitative tests. If the possible coloration of the resurgences is not objectionable, the estimated amount of dye should be increased. Care must be taken to keep maximum tracer recovery concentrations below the visible threshold (about 30 μg/L for sodium fluorescein) if the tracer could affect a water supply. The US Geological Survey suggested the maximum dye concentration of 10 μg/L at water intakes, but this is a conservative number and is not based on toxicological considerations (Alexander and Quinlan, 1996). The monitoring locations should include all possible resurgences and one or two “impossible” ones to serve as controls for the test. Carbon detectors are usually made of envelopes of plastic screening material containing a few grams of activated carbon granules (about 6–14 mesh). Cotton detectors are usually balls of unbleached cotton. The passive detectors should be changed at regular intervals, usually between 2 and 14 days. The detectors should be in place for at least one changing interval prior to dye injection to establish background information. They should be placed to remain submerged with changing water levels, be readily recoverable, and remain out of sight to causal observers. An interesting “hanger” for detectors in deeper water was developed by James Quinlan and is shown in Fig. 2. The detectors should be placed so medium velocity current flows through them and they are not exposed to direct sunlight. Detectors lose their ability to adsorb the tracers through time, but they will retain the dye for periods of at least 1 year following exposure when kept in total darkness. Caution must be used to prevent contamination of the detectors and they should be placed before the dye is injected. Injection points for the tracer are ideally flowing streams at surface sink-points or upstream of impassible reaches in a cave. Powered dyes are often mixed with water prior to injection to avoid unwanted spread of the dye. The injection points should be chosen to minimize the surface exposure of the tracers to sunlight. Tests involving injection points without rapid continuous flow such as a dry sinkhole or water well are much more difficult and more prone to failure. Tracer tests from sinkholes or wells are usually only conducted by professional hydrologists. The basic procedure is to use a tank truck to deliver water to the site. Several thousand liters of water are run into the sinkhole before the dye is added. The dye is then flushed into the aquifer system with about an additional 10,000 L of water. Injection sites that use water wells often provide unsuccessful tests, which mean that the dye injected as the tracer will not be usable in the area for a number of years. Caution and careful planning are needed for this type of test. Another dye injection involves leaving the dye in a safe storage area in a presently dry sink or streambed and letting the next storm wash the tracer into the flow system. This “dry set” system works best in remote areas where people or animals are unlikely to encounter the tracer before it is removed by precipitation. The analysis of the carbon for the presence of sodium fluorescein involves the following steps: 1. Rinse the carbon in clean water to remove sediment 2. Fill a test tube about half full with the carbon
FIG. 2 A “Quinlan Gumdrop” anchor for passive dye collectors. The base is concrete molded in a coffee can and the detectors are suspended above the bottom of deeper springs or streams. The screen envelops contain unbleached cotton (right arm) and activated carbon (left arm and bottom). The packets can also be secured with wire to rocks or other solid anchors such as old railroad spikes.
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3. Pour in the test solution, called the eluent, to cover the carbon about 3 cm deep (5–10 mL) 4. Let the samples sit undisturbed in the dark for between 30 min and 24 h 5. Shine a concentrated white light source such as a focusable flashlight through the test tube and look for the characteristic fluorescent green sheen on top the carbon granules (do not shake or disturb the solution for a visual examination) Most of the tests conducted today use a fluorometer to analysis the elutant rather than relying on visual examination as described above. Some workers prefer to use a larger diameter vial to increase the length of the light path through the elutant. The most commonly used eluent for sodium fluorescein is 5% potassium hydroxide (KOH) in 70% isopropyl alcohol. Many other eluents have been used including the “Smart Solution” (50% 1-propanol, 20% NH4OH, and 30% distilled water by volume). It takes some practice to distinguish weakly positive sodium fluorescein tests from some naturally occurring background. Numerous trials at varying dye concentrations should be run under controlled conditions before moving on to actual field tests. One of the advantages of the passive collector system is that the samples are time integrated for the exposure period of the detectors (Fig. 3). The amount of dye adsorbed and subsequently eluted from the carbon detectors is a function not only of the concentration of the dye passing the sampling site but the exposure time to the carbon, the surface area of the carbon in contact with the dye and the flow rate through the detector package. The lab protocol, especially the time allowed for the elution process, also affects the final apparent dye recovery concentration from the carbon. Some researchers (Field, 2002) object to the nonquantifiable nature of tests conducted using passive detectors for monitoring but the advantages in terms of being able to detect dyes at recovery concentrations that would be below the detection limits for water samples and the cost efficiency make the used of carbon detectors a primary tool for tracer tests (Aley, 2016).
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Time (days) FIG. 3 Bar graph illustrating the dye concentration integrating effect from carbon detectors compared to discrete water samples. Carbon detectors were changed at 24-h intervals and water samples were collected at the time the carbon detectors changed.
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Quantitative water tracing in karst areas Many of the professionally conducted water tracing tests are quantitative. Resurgences are sampled at short discrete time intervals and the water is analyzed using a calibrated fluorometer to determine the concentration of the dye in each sample. Several different dyes may be injected simultaneously so multiple sink points may be tested at the same time and under the same flow conditions. The results are plotted on a graph showing the concentration of the dye through time (breakthrough curve). The expense and complexity of quantitative tests are much higher, but considerably more information is obtained about the nature of the subsurface flow systems. The combination of quantitative dye tracing results with discharge measurements allows a more certain delineation of karst drainage basins and the calculation of the percentage of discharge from different sources at a given resurgence. Good results are dependent on using an appropriate tracer, adequate sampling frequency, and accurate analysis of the samples. A paper by Field (2003) provides formulas for determining dye mass and sampling frequency, but a good estimation of hydraulic and transport characteristics of the system is needed. This usually requires advance knowledge of the flow conditions and the resurgence(s). An estimate of the expected travel time from sink to resurgence is also helpful in planning tests. Flow velocities for conduit karst aquifers are typically in the range of 0.2–7 km/day, but some tracer tests have shown results far outside of these values. The planning and dye injection procedure is essentially the same as described for qualitative traces. Qualitative tests are often conducted first to identify which resurgences to monitor and to estimate the travel time for the tracer. Most tracer tests in karst areas use an instantaneous injection of the dye, but continuous (over several hours) releases are occasionally used to facilitate the calculation of discharge at the recovery point. Fluctuations in background fluorescence are often a problem in the blue and green wavelengths, so the red dye Rhodamine WT is the most commonly used fluorescent dye for quantitative studies. Quantitative tests generally require about twice as much dye as qualitative tests. The much greater sampling effort required for quantitative testing is the major reason for the increase in cost. The instrument used to determine fluorescent dye concentrations is a fluorometer (Fig. 4). Fluorometers pass light through the sample at a controlled excitation wavelength and measure the relative fluorescent intensity of the output signal at the emission wavelength. Laboratory standards at different dye concentrations must be prepared for each batch of dye to calibrate the fluorometer to determine dye concentrations. Filter fluorometers may be laboratory or field instruments and use various filters to set the excitation and emission wavelengths, while scanning spectrofluorophotometers are laboratory instruments that are better suited to analyzing samples containing multiple dyes or having high fluorescent backgrounds. A continuous scan through the wavelengths of interest with about a 20-nm separation between the excitation and emission settings allows a single scan to be analyzed for several different dyes. Some workers use computer enhancement of the instrument output signal to help identify and separate overlapping fluorescent peaks. Spectrofluorophotometers are more selective in their ability to isolate specific fluorescent compounds, but both types of fluorometers are highly sensitive to low concentrations of dye. Recording filter fluorometers provide a high-resolution record of the passage of the dye pulse, but spectrofluorophotometers are more reliable in the identification and quantification of individual tracers. Filter fluorometers may be fitted with a flow-through door which circulates water past the instrument’s light source. This provides a continuous record of fluorescent intensity at one set of wavelengths, but this is usually only practical for tests with travel times of less than 24 h. Fluorometric probes compatible with data loggers make continuous fluorometric measurements more cost-effective for many field studies and record a nearly continuous picture of the dye pulse-through time. Handheld
FIG. 4 A scanning spectrofluorometer used for laboratory quantification and identification of fluorescent dyes and a battery powered hand-held fluorometers (lower left) used for field monitoring of a dye pulse.
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fluorometers with several “channels” set for specific tracer dyes are robust enough to take into caves, but the samples must be taken manually. Most quantitative tracer studies at present involve the use of automatic water samplers at several resurgences over a period of days to weeks. The sampling interval is dependent on the expected travel time of the tracer and may range from 15 min to 1 per day. Some resolution in the shape of the dye recovery curve may be lost if the sampling interval is too long. Sampling should continue for a time sufficient to ensure that most of the dye pulse has been recovered. Samples should be stored in the dark in glass containers to minimize photochemical decay and sorption of the dye to the sample bottles.
Interpretation of water tracing tests Qualitative tracer tests are positive if the dye is detected at one or more monitoring points. A successful tracer test may prove a hydrologic connection between the injection point and recovery point(s) of the tracer, but the exact flow path(s) will remain unknown. Depending on the changing interval for the detectors, some information of the travel time for the tracer may be available. If the tracer is not detected at a monitoring station, it is listed as “none detected” rather than “negative.” Negative tests do not prove that a connection does not exist and the percentage of the injected tracer actually recovered can only be determined by quantitative measurements of the dye concentration and discharge at the resurgences during the recovery period of the dye pulse. Tracer tests are representative of the karst flow system for the hydrologic conditions prevailing at the time of the test. Overflow routes may exist that function only during high-flow conditions, so a thorough study of a karst region will require multiple tests at different water levels. Some resurgences may only function during high-flow conditions. The travel times for the tracers will be less at high water levels, and the dye will move much faster if the conduits are completely flooded. Quantitative tests between the same injection and recovery sites under a range of flow conditions may be used to develop the relationship between travel time and groundwater discharge and to predict solute transport characteristics. Discharge-dependent thresholds may be identified. The graph of dye recovery concentration plotted through time is called a dye recovery or breakthrough curve (Fig. 5). This curve presents a picture of the passage of the dye “cloud” or pulse-through time at the stationary sampling point. Breakthrough curves are typically skewed to the right with rapid rise to peak (maximum) dye concentration and a long trailing edge of declining dye concentrations. The shape or form of the breakthrough curve is a function of the characteristics of the tracer, the flow conditions during the test, and the characteristics of the aquifer or conduit system. The mean flow velocity is usually computed using the straight-line distance between sink and resurgence and the time between dye injection and recovery of half of the tracer. This is the “centroid” or center of recovered mass on the breakthrough curve. Some workers use the time to peak concentration if the resurgences are not completely sampled through the long trailing period of declining tracer concentrations. If discharge is known for throughout the dye recovery period, the amount of dye recovered can be calculated by integrating the area under the breakthrough curve and multiply by the discharge: 25
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Hours from dye injection FIG. 5 Graph showing a simple breakthrough curve from a tracer test from a cave to a spring. The principal components of the curve are (1) first arrival of the dye at 30 h; (2) peak (maximum) recovery concentration at 42 h; (3) dye centroid (center of mass and median recovery point) at 50 h; and (4) total time of dye cloud passage from 30 through 120 + hours. Note the 4-h sampling interval and that sampling was stopped before the complete passage of the dye cloud.
Water tracing in karst aquifers Chapter 134
FIG. 6 Graph showing dye recovery (red line) and the percent recovery of injected dye (dashed blue) line through time. Sampling was terminated before the passage of the dye cloud was complete but it is probable that this spring accounts for all the water from the tracer injection site.
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where M ¼ mass of dye recovered. Q ¼ discharge. C ¼ dye concentration at time t. Discharge is usually based on current-meter measurements during the test. If the tracer is conservative (not subject to decay or sorptive losses) and all of the water from the injection point is accounted for, the mass of dye recovered should be equal to the mass injected. This is the dye budget for the test (Fig. 6). None of the fluorescent tracers are completely conservative, but tracer tests with travel times of less than a few days should balance reasonably well if all of the resurgences have been monitored. Assuming complete mixing and a conservative tracer, the area under the time-concentration breakthrough curve should be constant even as the shape of the curve changes in the downstream direction. A significant loss of the tracer from the system suggests that additional resurgences are present or some of the tracer has moved into storage areas along the flow route. Less conservative tracers usually produce a longer travel time estimate due to retardation caused by sorption and desorption along the flow route. Breakthrough curves for many tracer tests in karst areas exhibit multiple peaks, and the time between the peaks may be dependent on discharge. The interpretation of these tests is somewhat subjective, but much insight into the internal flow characteristics of the karst aquifer and the conduit system can be gained from these tests. The best breakthrough curve data are obtained using continuous flow fluorometry during the passage of the dye pulse. The shape or form of the dye cloud is dependent on the interaction of dispersion, dilution, divergence and convergence, and storage (Smart, 1988). The quantitative interpretation of breakthrough curves is generally based on solute transport equations. An introduction to this type of analysis was presented in Benischke et al. (2007) and a computer program to analyze tracer recovery data was developed by Field (2002). Traces involving longer distances and longer travel times show lower peak concentrations and longer persistence of the dye pulse as the tracer becomes increasingly more dispersed with time and distance. The shape of the tracer cloud becomes more asymmetrical as dispersion is influenced by the tracer moving in and out of pockets of “dead zone storage” along the flow route. Dispersion is also affected by the retardation factor for tracers that are less than completely conservative. Divergence occurs where the underground flow routes divide or break away from the main conduit. Convergence is where the routes rejoin the main conduits. The divergent routes generally are longer flow paths than the main conduit, so the portion of the flow and tracer diverted may reappear as second or multiple pulses at the sampling station. Dilution is caused by unlabeled tributary water mixing with the water containing the original tracer injection. This will cause a lower concentration of dye in the downstream water samples. Storage of the tracer in the conduit system may be very complex. Storage may just represent very low-velocity flow-through one section of a branching conduit, called in-line storage. A test done under conditions of diminishing flow may leave some of the
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FIG. 7 Sketches showing conceptual models of possible dye breakthrough curves. The pattern of the recovery curve is a function of prevailing water levels at the time of the test and the geometry of the conduit system. (From W.K. Jones, W. K., 1984. Analysis and interpretation of data from tracer tests in karst areas. Natl. Speleol. Soc. Bull. 46 (2), 41–47.; after Smart, C. C., Ford, D. C., 1982. Quantitative dye tracing in a glacierized alpine karst. Beitr. Geol. Schweiz-Hyrdrol., 28(1), 191–200. With permission.)
dye abandoned in upper level pockets to be remobilized when the flow increases at a later date. Some dye may become stored offline in lateral areas along the route such as small voids or in pores in the sediments. In-line storage tends to create dye pulses that correlate with flushing due to storm events. Dye from off-line storage tends to drain slowly during groundwater recessions. Some conceptual models of breakthrough curves for different flow levels and conduit geometries are presented in Fig. 7 (adapted from Smart and Ford, 1982). The best picture of the conduit aquifer system for any karst area is from the actual surveying of the cave passages. Tracer tests, interpreted in the context of the regional geology and hydrologic setting, can be used to fill in the blank place on the map where actual exploration of the caves is not possible. The rather unusual mixture of quantitative techniques and subjective interpretation of the unseen part of the aquifer makes water tracing a fascinating endeavor.
See also the following articles Hydrogeology of Karst Aquifers
Bibliography Alexander Jr., E.C., Quinlan, J.F., 1996. Practical tracing of groundwater with emphasis on karst terranes. In: Guidelines for Wellhead and Springhead Protection Area Delineation in Carbonate Rocks (prepared by Eckenfelder Inc.), pp. 1–38. EPA 904-B-97-003, Appendix B. Aley, T., 2016. Using activated carbon samplers to improve detection of fluorescent tracer dyes in groundwater remediation studies. In: Rectanus, H.V., Rodgers, P. (Eds.), Remediation of Chlorinated and Recalcitrant Compounds. Tenth International Conference on Remediation of Chlorinated and Recalcitrant Compounds, A-056. www.battelle.org/chlorcon. Behrens, H., Beims, U., Dieter, H., Dietze, G., Eikmann, T., Grummt, T., Hanisch, H., Henseling, H., Kass, W., Kerndorff, H., Leibundgut, C., Muller-Wegener, U., Ronnefahrt, I., Scharenberg, B., Scharenberg, B., Schleyer, R., Schloz, W., Tilkes, F., 2001. Toxicological and ecotoxicological assessment of water tracers. Hydrogeol. J. 9, 321–325.
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Benischke, R., Goldscheider, N., Smart, C., 2007. Tracer techniques. In: Goldscheider, N., Drew, D. (Eds.), Methods in Karst Hydrogeology. Taylor and Francis/ Balkema, pp. 147–170. Field, M.S., 2002. The QTRACER2 Program for Tracer-Breakthrough Curve Analysis for Tracer Tests in Karstic Aquifers and Other Hydrologic Systems. U.S. Environmental Protection Agency. 600/R-02/001. 179 p. Field, M.S., 2003. A review of some tracer-test design equations for tracer-mass estimation and sample collection frequency. Environ. Geol. 43, 867–881. Ford, D.C., Williams, P.W., 2007. Karst Hydrogeology and Geomorphology. p. 195, Wiley, West Sussex. Freixes, A., Monterde, M., Ramoneda, J., 1997. Tracer tests in the Joeu karstic system (Aran Valley, Central Pyrenees, NE Spain). In: Kranjc, A. (Ed.), Tracer Hydrology 97. A. A. Balkema, Rotterdam, The Netherlands, pp. 219–225. Gabrovsek, F., Kogovsek, J., Kovacic, G., Petric, M., Ravbar, N., Turk, J., 2009. Recent results of tracer tests in the catchment of the Unica River (SW Slovenia). Acta Cardiol. 39 (1), 27–37. Goldscheider, N., Meiman, J., Pronk, M., Smart, C., 2008. Tracer tests in karst hydrogeology and speleology. Int. J. Speleol. 37 (1), 27–40. Jones, W.K., 1984. Analysis and interpretation of data from tracer tests in karst areas. Natl. Speleol. Soc. Bull. 46 (2), 41–47. K€ass, W., 1998. Tracing Technique in Geohydrology. A. A. Balkema, Rotterdam, The Netherlands. 581 pp. Mull, D.S., Liebermann, T.D., Smoot, J.L., Woosley Jr., L.H., 1988. Application of dye-tracing techniques for determining solute-transport characteristics of ground water in karst terranes. In: U.S. Environmental Protection Agency, EPA. p. 103 904/6-88-001. Smart, P.L., 1984. A review of the toxicity of twelve fluorescent dyes used for water tracing. Natl. Speolol. Soc. Bull. 46 (2), 21–33. Smart, C.C., 1988. Artificial tracer techniques for the determination of the structure of conduit aquifers. Ground Water 26 (4), 445–453. Smart, C.C., 2005. Error and technique in fluorescent dye tracing. In: Beck, B.F. (Ed.), Sinkholes and the Engineering and Environmental Impacts of Karst. American Society of Civil Engineers, Geotechnical Special Publication 144, pp. 181–200. Smart, C.C., Ford, D.C., 1982. Quantitative dye tracing in a glacierized alpine karst. Beitr. Geol. Schweiz-Hyrdrol. 28 (1), 191–200. Smart, P.L., Laidlaw, I.M.S., 1977. An evaluation of some fluorescent dyes for water tracers. Water Resour. Res. 13, 15–33. Taylor, C.J., Greene, E.A., 2008. Hydrogeologic characterization and methods used in the investigation of karst hydrology. In: Rosenberry, D.O., LaBaugh, J.W. (Eds.), Field Techniques for Estimating Water Fluxes Between Surface Water and Ground Water, pp. 75–144. Chapter 3. White, W.B., Herman, J.S., Herman, E.K., Rutigliano, M., 2018. Karst Groundwater Contamination and public health. Advances in Karst Science, Springer. 347pp. Wilson, J.F., 1968. Fluorometric Procedures for Dye Tracing. Techniques of Water-Resources Investigations of the U. S. Geological Survey, Book 3. Chapter A12 (Revised). pp. 1–34.