Artificial tracer tests

Tracer tests are a powerful tool in karst hydrogeology to investigate groundwater flow in fast-draining conduit systems. To trace groundwater movement, artificial tracers are injected into the aquifer and the spread of the tracer plume is monitored at surrounding sampling points. In this way, tracer tests deliver specific information about point-to-point connections and catch-ment areas of springs, underground drainage pattern and flow paths, and transit-time distribu-tions and flow velocities in karst systems (Käss 2004; Goldscheider and Drew 2007). In con-junction with detailed geologic and hydrologic information, the underground drainage of a karst aquifer system can be characterized.

Fluorescent dye tracers are often used as artificial tracers because their solubility, chemical stability and low adsorption properties facilitate dilution and transport in groundwater. Uranine is an almost ideal tracer as, in comparison with other dyes, it has the lowest adsorption proper-ties and the lowest detection limit – 0.005 µg/L (Käss 2004; Goldscheider and Drew 2007). The tracers sulforhodamine G, eosine, sodium naphthionate and tinopal are often used as additional

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dyes when conducting multi-tracer tests with several tracer injections. Because of their charac-teristic fluorescent wavelength, the dye tracers are clearly detectable in water samples. High-precision analytical laboratory results are obtained using a fluorescent spectrometer (Perkin Elmer, LS50B / LS55) and the syncho-scan method. Field fluorimeters (Albillia, GGUN-FL 43, 334 and 335) can be used for continuous detection, providing high temporal resolution of tracer concentrations. Cumulative and qualitative detection of tracers is possible with charcoal bags.

Most tracer tests are conducted with tracer injections on the land surface and the tracer is flushed into the karst system using natural seeping water, e.g., meltwater, or artificial irrigation, e.g., water tanks (Fig. 2.2). Under these conditions, the tracer seeps gravitationally through the un-saturated zone of the aquifer and follows the hydraulic gradient through the un-saturated zone to the sampling point. Results of hydraulic parameters characterize groundwater drainage along the whole flow path and allow estimates for flow velocities (v) and transit times (t) in the con-duit system. The shape of the observed breakthrough curve (BTC), i.e., the peak and the tailing, provide information about the flow path (Field and Nash 1997; Massei et al. 2006). While al-most symmetrical BTCs are indicative of well-developed conduits, highly asymmetrical and right-skewed BTCs indicate retardation and storage processes in the unsaturated zone as a result of interaction between the karst conduits and the fissured rock matrix.

Figure 2.2: a) Point-to-point connection between injection point and sampling points, b) schematic profile between the injection point and sampling point at the land surface, and c) observed breakthrough curve at the spring.

A specific tracing technique is possible where active karst conduits are accessible (Goldscheider et al. 2008). In contrast to classical applications, tracer tests with injection and monitoring in active caves have been used to determine variable flow parameters for different zones of the aquifer, i.e., vadose, epiphreatic and phreatic zone (Fig. 2.3) (Hauns et al. 2001). It has been shown by Meiman et al. (2001) that tracer tests can contribute to resolving the internal structure of the karst drainage network and to identify sub-catchment areas. Because of the logistical challenges of working in caves and the often high associated effort and costs, in-cave tracer

13 tests are not common (Perrin et al. 2007; Goldscheider et al. 2008). However, in collaboration with committed cave researchers, in-cave tracer tests offer a unique opportunity to observe un-disturbed groundwater flow in karst systems.

Figure 2.3: In-cave tracer tests make it possible to obtain a) spatially resolved information about the flow path from injection point to the spring, b) spatially resolved information about conduit flow in the epiphreatic and phreatic zones of the aquifer and c) temporally resolved information can be obtained by analyzing breakthrough curves at all sampling points.

Hydrologic flow conditions can affect groundwater flow in karst areas. It has been shown by Göppert and Goldscheider (2008) and Pronk et al. (2007, 2009) that transit times and dilution of the tracer are highly variable under high-flow and low-flow conditions. Furthermore, under-ground drainage divides and catchment areas can shift depending on water levels in the system (Ravbar et al. 2011). Therefore, tracer tests conducted under different flow conditions can pro-vide insight into different flow parameters across the temporal variations of underground drain-age.

For a quantitative evaluation of tracer tests and the determination of transport parameters, such as longitudinal dispersion coefficient (D), BTCs can be modeled by different advection-disper-sion models (ADM) (Field and Pinsky 2000; Geyer et al. 2007; Massei et al. 2006; Morales et al. 2007; Goldscheider et al. 2008). The models account for one-dimensional flow that is con-trolled by advective and dispersive transport processes in the direction of groundwater flow (Eq. 2.1). To solve the general transport equation, simplifying assumptions are necessary, such as homogenous flow, a uniform and unidirectional flow field that is constant in time and space, and constant flow parameters along the flow path (van Genuchten et al. 2012). An inverse mod-eling tool of the ADM provides best estimates of the two flow parameters (v, D) by fitting a modeled BTC to observed values.

= − (2.1)

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For the evaluation of nearly symmetric to slightly skewed BTCs, the advection-dispersion model implemented in the program CXTFIT (Toride et al. 1999) is used to obtain flow param-eters of the karst conduit system. As highly asymmetric and right-skewed BTCs are character-ized by a strong interaction between conduits and the rock matrix, the application of a multi-dispersion model, as implemented in TRACI95 (Käss 2004), is required to obtain flow param-eters for the fast drainage of the conduits and the intermediate drainage at the margins of the conduit system.