Groundwater Tracing

Floridan Aquifer
The Floridan aquifer encompasses all of the water bearing limestone rocks in Florida that contain the underwater cave systems familiar to cave divers around the world including the massive caves in the Woodville Karst Plain. Hydrogeologically speaking, the Floridan aquifer is in a special class of aquifers known as karst, which, loosely defined, describes aquifers made of soluble rock wherein groundwater flow dissolves preferential flow paths (caves) that then control the flow rate and direction. Unlike most of the rest of the karst aquifers in the world (e.g. the limestone rocks containing the big dry caves in the Appalachian Mountains, Mammoth Cave, Carlsbad Caverns, etc.), the Floridan aquifer is further unique because the limestone rocks comprising the aquifer are themselves highly permeable. This means that groundwater, in usable quantities, can be found almost anywhere in Florida, not just in the caves, as is most often the case in other karst settings. 

The unique hydrologic conditions that characterize the Floridan aquifer (karstic features within a highly permeable aquifer matrix) have been both blessing and curse to the preservation of Florida's natural resources. Blessing, because development has not, until the recent boon in the water bottling industry, needed to focus on Florida's springs or rivers as a major source of fresh water. There are very few reservoirs in Florida, 

Figure 1.  Generalized conceptualization of the hydrogeologic cycle showing water in constant circulation between the atmosphere, surface water, and groundwater. (A) Most common conceptualization of the groundwater reservoir indicating that indicating that aquifers are an essentially uniform mass of porous material wherein groundwater moves very slowly down-gradient toward lakes, rivers, and streams. (B) Refined conceptualization of karst aquifers wherein dissolved conduits and caves carry the majority of the groundwater very rapidly to spring discharges.

Poor Scientific Assumptions
Unfortunately, the same conditions that have enabled development to proceed nearly oblivious to abundant water resources provided by Florida's springs and rivers has also been, and continues to be, a curse to resource protection efforts in more subtle ways. The curse arose and flourishes on a fundamental misunderstanding of the significance of caves and other karst features in Florida to groundwater flow patterns and velocities. Basically, because highly productive wells can be drilled at almost any location in Florida, hydrogeologists and water resource managers, most having never been in an underwater cave in Florida, were quick to assume that such features were anomalous and not pertinent to groundwater flow patterns. This was an understandable assumption given the comparison of Florida to other, recognized, karst settings where water wells are typically either dry or deliver very low volumes of water when they do not intersect caves or dissolved fractures. In a practical sense, the simplifying assumption applied to the Floridan aquifer is that groundwater flow is largely diffuse and through a porous media with a few discrete discharge locations (springs); rather than the more complicated, though valid condition, where flow through the diffuse media converges on progressively larger conduits that deliver the water to the springs (Figure 1). This assumption persisted largely unchallenged until recently when the decline in spring water quality across Florida due to increased nitrate levels became all but undeniable and water resource managers were forced to consider that the traditional assumptions about the Floridan aquifer might be wrong; that caves do exist, are more common than earlier assumed, and are probably important pathways for contamination.

Figure 2: Map of the Woodville Karst Plain, north central Florida showing the location of the Leon Sinks cave system in the north and Wakulla cave in the south relative to the position of the Cody Scarp, which demarks the transition from confined aquifer conditions in the north to unconfined conditions in the south. Note that the potentiometric surface lines, which are drawn assuming that there are no caves in the region, indicate a general flow to the south rather than concentrated flow in the caves.

The Woodville Karst Plain
The Woodville Karst Plain (WKP) of north Florida, though it contains two of the largest springs in Florida and two of the largest and longest mapped underwater cave systems in the world, is one of the many karst basins in Florida where the simplifying assumption about groundwater flow has led to a poor understanding of the realities of groundwater flow patterns, rates, and their impact on spring water quality. 

The WKP is a broad lowland that extends from just south of Tallahassee to the Gulf of Mexico, where the Floridan aquifer is either unconfined or poorly confined (Figure 2). The region is underlain by numerous very large underwater cave systems that convey water from upland recharge areas to spings such as Spring Creek and Wakulla springs, the largest and third largest springs in Florida respectively, and Indian, Sally Ward, and Mc Brides springs. Underwater cave surveys conducted by the WKPP indicate that the Leon Sinks cave system, alone, contains more than 18 km of saturated conduits that convey groundwater generally north-south through a series of 26 sinkholes or "karst windows" toward the Gulf of Mexico. The up-gradient most sinkholes are located in the Leon Sinks Geological Park in Leon County. The down-gradient most sinkhole is Turner Sink located in Wakulla County northwest of the town of Bethel between State Road 267 and State Road 61. A large conduit approximately 15 m in diameter has been explored and mapped for approximately 1.2 km further down-gradient and appears to continue without any diminishment of size or flow (Jablonski, personal communication). Discharge from the system is thought to occur at either at Wakulla spring or, more probably, the Spring Creek springs located on the Gulf Coast. Wakulla Cave contains an additional 10-12 km of underwater conduits, the largest of which trend for more than 6 km south of Wakulla Spring (Kincaid, 2000). Conduit diameters throughout these caves range from less than 2 meters to more than 30 meters and average approximately 10-15 meters. The average depth of the conduits is approximately 85 meters below the water table surface. Flow rates through the conduits have not been directly measured, however the velocities are sufficiently large to impede a divers ability to swim against the flow direction even in the largest passages. 

Considering these characteristics, it becomes evident that the caves are the most significant hydrogeologic features in the WKP, however there remains very little substantive scientific information about the caves or how they affect groundwater flow patterns in the region, or how they contribute to the increasing water quality problems at the springs. To address this deficiency, the Florida Geological Survey initiated a comprehensive research program aimed at characterizing groundwater flow paths through the WKP in the summer of 2002 that includes the tracer testing efforts described in this paper.

Artificial Tracers
Groundwater tracing with artificial tracers involves adding a label to the groundwater that can be identified if that same water is sampled at a different location. Tracing involves making almost no assumptions about an aquifer's hydraulic properties; the principal assumption made is that the tracer can be identified when it is recovered. When used properly, artificial tracers, therefore, provide the most reliable data on groundwater flow patterns and velocities that can be obtained. The most commonly used artificial tracers are fluorescent dyes such as: Rhodamine WT (red), Eosin (green), Uranine (green), and optical brighteners (various colors), however many other substances have been used including: chlorine (salt), low level radioactive compounds, bacterial phage, microspheres (tiny plastic balls), ping-pong balls, etc. Contaminant spills can also be used as artificial tracers if the source and timing of the spill can be confidently documented. The purpose of using artificial tracers is to answer three basic questions. 1. Where does the water go (say from a sinkhole) or where does the water come from (say from a spring)? 2. How long does it take to get there? 3. What happens to the water along the way?

Fluorescent Dyes
Fluorescent dyes are some of the oldest groundwater tracers known and have been used successfully for more than 100 years. A quantitative tracer test was done using the fluorescent tracer dye uranine in 1877 between the Rhine River and the Aäch Spring near the Swiss border, which showed that there was a reversal of flow of the Danube River across the European Continental Divide (Käss, 1998, p. 125). Since then more than 20,000 professionally conducted tracer tests using fluorescent dyes have been conducted worldwide, most of them in karst terrains. Fluorescent dyes are now the most commonly used groundwater tracers because they fulfill the following criteria for optimal artificial tracers:

1. they are readily soluble in water;

2. adequately conservative (they don't react with water or soil/rock);

3. unambiguously and inexpensively detectable at very small concentrations;

4. readily available and relatively inexpensive; and, most importantly

5. intrinsically low in toxicity thereby posing no significant health or environmental hazard (except for pink or green fingers). 

When a fluorescent dye is injected or released into a water body it mixes with the water, travels downstream with the flow and spreads due to molecular diffusion. At and near the point of injection, even very small quantities of a fluorescent dye will appear to stain the water yielding a vivid coloration (Figure 3). When small quantities are used however, the concentration of the 

Figure 3: Uranine dye moving downstream in Fisher Creek during a 2003 injection to trace the destination of the water disappearing at Fisher Creek.

Qualitative Tracer Tests
There are two basic types of dye tracer tests: qualitative and quantitative. Qualitative tests are the simplest, most common, but least informative. They are designed to answer the basic question of connection. One or more dyes are injected at an upstream location(s). Samples are collected at downstream locations. If the dye is observed, a connection is verified and an approximate travel time can be determined. Typically, qualitative tests are conducted using some type of dye trap, such as activated charcoal or cotton, at the sampling locations. The traps, or bugs, are often left for long periods of time, after which any dye is eluded from the absorptive material into a water sample and tested with a fluorometer to determine if any dye is present in the sample. Simplicity is the most significant advantage of a qualitative test wherein many locations can be sampled with minimum resources. Problems with quantitative testing revolve around control because, over broader time periods, it is often impossible to ensure that your dye is the only fluorescent substance introduced to the flow system or that small spikes in fluorescence are not simply due to fluctuations in the background levels.

Quantitative Tracer Tests
Quantitative tests, on the other hand, are more complex to design, more labor intensive to conduct, but deliver far more useful and reliable information. A quantitative test depends not only on the detection of a fluorescent substance at the sampling locations but rather on the observation of the trend in fluorescence (from background levels to peak concentration and back to background levels) as the dye passes the observation point. The observations are recorded on a plot of the fluorescence or dye concentration levels over time, which is often called a breakthrough curve (Figure 4). The breakthrough curve offers many benefits over a single test including:

• observation of the increase and decrease in fluorescence at the sampling point increases confidence in the assumption that the samples reflect passage of the injected tracer rather than some other anomalous source of fluorescence;

Figure 4: Example breakthrough curve for an injected dye tracer. The width of the curve provides information about dispersion and sinuosity of the conduit flow path. Groundwater velocities are calculated from the time at which the peak concentration passes the sampling station.

• a more accurate groundwater velocity can be calculated using the time-to-leading edge and time-to-peak concentrations; and

• evaluating the shape and integrating the trend of the concentration vs. time plot provides for the calculation of other hydraulic parameters including: longitudinal dispersion, Reynolds and Peclet Numbers, and discharge. 

Quantitative tracers tests can be conducted wherein recovery curves have a peak concentration of less than one part per billion, which is far below the visible detection limit. However, in order to achieve successful tests, sampling strategies must be designed such that numerous samples are collected from within the dye cloud as it passes a sampling point. Such sampling strategies are typically laborious and their design often requires a fair amount of professional judgment and luck.

Natural Tracers
Natural tracing involves the use of naturally occurring components of a water sample to determine information about the source and age of the sample. The most commonly used natural tracers are isotopes and chemical compounds that originate in the atmosphere and become incorporated in the rainfall the recharges an aquifer. Of those types of substances, the isotopes of oxygen (¹⁶O/¹⁸O) and hydrogen (²H/³H), and chlorofluorocarbons (CFC) are the most commonly used tracers today. Isotopes of other elements such as Radon, sulfur, chlorine, lead, strontium, helium and carbon are other notable tracers that are fairly common in hydrogeology. 

In the case of age-dating, the basic idea behind the use of natural tracers is that of a two-component mixing model, wherein the concentrations of a given substance can be fairly well constrained in two separate source waters and some value in between the two end members represents a mixture of only those two waters. Knowing something about decay rates or linear changes in the source concentrations provides for an estimation of age, or time since recharge. 

For example, tritium (³H), which has been one of the most commonly used radiogenic isotopes, was released to the atmosphere in large quantities during the atmospheric nuclear testing program, which largely began in the 1950's and peaked in the early 1960's. During the peak period, atmospheric nuclear explosions typically released several hundred tritium units (TU) to the atmosphere that was eventually incorporated into precipitation. The dramatic increase in atmospheric TU during this period is often referred to as the "bomb spike" and is the reason that TU can be used as an effective groundwater tracer. 

After the end of the atmospheric testing program the source of tritium to the atmosphere was eliminated and, because tritium has a relatively short half-life (12.3 years), there has been a predictable decline in TU concentrations in groundwater. After accounting for radioactive decay, water with TU values similar to atmospheric levels during the bomb spike can be attributed to recharge during that period. As time goes by, the usefulness of TU as a groundwater tracer will decline because radioactive decay will render the TU values of bomb spike water similar to the current atmospheric levels. Raw TU values continue to be useful as a tracer because the tritium activity has not yet decayed away to completely undetectable amounts, although in many parts of the country, including Florida, the TU values are approaching undetectable levels (Clark and Fritz, 1997). 

The main problem with the use of natural tracers to estimate the age of a water sample is the often erroneous, 2-component mixing model assumption, i.e. one source of recharge to one groundwater body. In the case of tritium, any TU number that is not either clearly related to the bomb spike or pre-bomb testing could be any mixture of several waters of different aquifer residence times. The utility of the TU as a quantitative tracer is thus severely limited in regions where groundwater discharge is known to be a composite of waters from multiple sources, which is likely the case in the karst regions of Florida. The same problem plagues all of the natural tracers as tools for calculating the age of any water sample especially in the Floridan aquifer, where spring discharges are known to be a mixture of, at very least, older matrix water and more recent recharge that enters the aquifer through discrete sources such as sinkholes. This problem will be explored in more detail in the discussion of tracing in the Woodville Karst Plain.

WHY IS TRACING IMPORTANT?
Florida's spring water quality has been declining markedly for more than 10 years, with the most notable problem being increasing nitrates. Within the last 5 years, the increasing nutrient levels has led to obvious and alarming changes in the water clarity and ecology of many spring basins. At Catfish Hotel, the most popular entrance to the Manatee Springs Cave System, dark brown algae covers the limestone walls in the sinkhole basin and near-entrance cave walls, which used to be clean rock surfaces. The same conditions are emerging at Devil's Eye and Ear, Blue Springs Orange City, Indian, Wakulla, and many more. If the trend continues, it is doubtful that any of us will ever again behold the air-clear conditions that used to typify most of Florida's springs and spring caves, as particulate levels continue to climb due to the increase in algae and bacteria in the water. 

Though these statements are fact, they are all too often dismissed as environmental alarmism or worse, inevitabilities. There is however, another perspective that serves as motivation for change. Consider that Florida 

Figure 5: Map showing the distribution of springs in Florida relative to aquifer confinement. Note that most of the springs cluster in the regions where the aquifer is either unconfined or thinly confined.

boasts the highest concentration of springs on the planet including 33 documented first-magnitude (very big) springs and more than 600 2nd and 3rd magnitude springs. Most of these springs are concentrated in a fairly small part of the state where the Floridan aquifer is unconfined (Figure 5); an area that extends like a crescent from Clearwater in the southwest up through the center of the peninsula and west across the panhandle to the Woodville Karst Plain. Until recently, most of these springs typically discharged crystal clear, pure fresh water to the land surface. People drank the water without treatment. Divers enjoyed visibility conditions limited only by the power of their lights. But, perhaps most significantly, people have long been coming from literally around the world to experience the springs because they know and have known what many Floridians have missed, that these springs make Florida unique, that such an abundance of clear fresh water cannot be found anywhere else on Earth. So, not only are we loosing our clear freshwater dive spots, we're systematically loosing the one place on Earth where such conditions could be found in abundance.

References

Clark, I.D., and Fritz, P., 1997, Environmental Isotopes in Hydrogeology, Lewis, New York,  328 p. 

Jablonski, J.M., 2002, personal communication; Global Underwater Explorers, 15 south Main St., High Springs, Florida; phone (386) 454-0811.

Käss, W., 1998, Tracing Technique in Geohydrology, Balkema, Rotterdam, 581 p.

Further Reading

Cao, H., J. Cowart, and J. Osmond, n.d., Water Sources of Wakulla Springs, Wakulla County, Florida: Physical and Uranium Isotopic Evidences, Tallahassee, FL, 12 pages.

Field, M., R. Wilhelm, J. Quinlan and T. Aley, 1995, An Assessment of the Potential Adverse properties of Fluorescent Dyes used for Groundwater Tracing, Environmental Monitoring and Assessment, vol.47 pp. 1-21.

Field, M. and S. Nash, 1997, Risk Assessment Methodology for Karst Aquifers: (1) Estimating Karst Conduit-flow Parameters, Environmental Monitoring and Assessment, vol.35 pp. 75-96.

Hisert, R.A., 1994, A multiple tracer approach to determine the ground and surface water relationships in the western Santa Fe River, Columbia County, Florida, Ph.D. Dissertation, University of Florida, Gainesville, Florida.

Katz, B., T. Coplen, T. Bullen, and J. Davis, 1997, Use of Chemical and Isotopic Traces to Characterize the Interactions between Ground Water and Surface Water in Mantled Karst, Groundwater, vol. 26, no. 6, pp.1014-1028

Katz, B., J. Catches, T Bullen, and R. Michel, 1998, Changes in the isotopic and chemical compostion of ground water resulting from a recharge pulse from a sinking stream, Journal of Hydrology, Vol. 211, pp. 178-207.

Kincaid, T., 1998, River Water Intrusion to the Unconfined Floridan Aquifer, Environmental & Engineering Geoscience, vol. 4, no.3, pp. 361-374.

Quinlan, J and A. Stanley, 1992, The Reponse of Landfill Monitoring Wells in Limestone (Karst) Aquifers to point sources and non point sources contamination, Conference on Hydrogeology, Ecology, Monitoring, and Management of Grounwater in Karst Terranes, Nashville, TN, Proceedings, National Groundwater Association, Dublin, OH, pp.529-540.

Skiles, W., Hayes, A.H., and Butt, P.L., 1991, Ichetucknee Hydrogeology Study, Florida Department of Natural Resources, Florida Park Service, District III, Gainesville, Florida.

Wilson, W.L. and Skiles, W.C., 1988, Aquifer characterization by quantitative dye tracing at Ginnie Spring, northern Florida, The Proceedings of the Second Conference on Environmental Problems in Karst Terranes and Their Solutions, The Association of Groundwater Scientists and Engineers, Nashville, TN.

Wilson, W.L., 1991, Dye Tracing Benefits and Practice in Florida, NACD Journal, vol. 23, no. 2.