Evaluation of the applied methods

To obtain hydrogeologic knowledge and quantitative data about hydraulic parameters in the different catchment areas, a combination of different approaches has been applied. The selec-tion of different investigating methods is highly dependent on the specific research focus, as particular techniques assess only specific drainage structures, e.g. conduits or fissured net-works. In this thesis, artificial tracer tests, analyses of stable isotopes, and evaluations of hy-drographs facilitated quantitative characterization of underground drainage properties.

Tracer tests with artificial tracers are a powerful tool in karst hydrogeology to assess conduit drainage. In conjunction with knowledge of the hydrogeologic setting and karst development, carefully designed tracer tests can provide valuable information for the delineation of catchment areas and the development of conceptual models. In this thesis, tracer tests allowed for the determination of transit times and flow velocities in the fast draining karst conduit system, which is crucial for groundwater management of the aquifer and the estimation of potential flow paths of contaminants. The method was especially suitable in areas that are difficult to access, i.e., alpine areas and caves, because the necessary equipment was manageable. Tracer tests were also conducted under different flow conditions, which enabled assessment of the dynamics of the drainage system for variable transit times and different effects of dilution in the individual karst systems. Additionally, it was possible to use a tracing technique that has rarely been applied in karst hydrogeology – tracer tests were conducted inside a cave system to obtain detailed insights into the structure of karst drainage and related flow parameters. Tracer injection and observation in accessible passages of the conduit system allowed for the resolution of flow parameters for epiphreatic and phreatic cave passages. In combination with injection on the land surface in remote parts of the catchment area, it was possible to determine the hier-archical structure of karst drainage. It can be concluded that such unique insights into the drain-age structure of karst aquifers were only possible from in-cave tracer tests. Although such tracer tests are laborious, the benefits are worth the effort since unique information about groundwater flow and flow parameters can be obtained.

The combined use of artificial and natural tracer methods was applied in this thesis to assess underground drainage properties in a fissured karst system. Fissured karst systems are charac-terized by the absence of an accessible conduit and cave system that prohibits in-cave tracer

125 tests to resolve the internal drainage structure. While the artificial tracing method provides in-formation about fast drainage structure, i.e., the conduit system, natural tracers generally deliver information about diffuse and slow flow paths. Even if data for isotopes in precipitation or at the springs are scarce, a simplified evaluation of stable isotopes is still possible using a lumped-parameter model. Such evaluation is especially important in areas that are difficult to access and where data collection is difficult, e.g., in alpine areas. Long-term data from nearby precip-itation stations and monthly water samples from springs provided sufficient input data. Data modeling with the program FLOWPC provided an evaluation of intermediate to slow transit times through the aquifer. The combination of artificial and natural tracers was suitable to de-velop a detailed conceptual model of the fissured karst system and to assess the fast and the slow drainage system. The approach resulted in 1) detection of three flow compartments com-prising a fast-, an intermediate- and a slow-flow component, 2) determination of the distribution of transit times in these three flow compartments, and 3) estimates of the contribution of the flow components to spring discharge. The results illustrate the triple porosity of the karst system and allow for estimation of available karst water resources.

To investigate a complex aquifer system comprising a karst and a porous-media aquifer, a com-bination of tracer tests and discharge analysis was conducted. Based on a conceptual model of the alluvial/rockfall system describing the hydrogeology and the surface water-groundwater interaction, the tracer test was used to determine subsurface transit times and flow velocities in the alluvial/rockfall system. Available long-term discharge data for two different sites in the valley allowed for the detailed evaluation of discharge characteristics. Results indicate that the following parameters were particularly suitable to describe the system: the discharge ratio, the recession coefficients, and the lag times of discharge peaks. In this study, the lag times between discharge peaks upstream and downstream in the valley was found to be a crucial parameter to describe the system. Sharp discharge peaks of the hydrographs were evaluated manually, while an impulse-response function was applied to evaluate the lag times of wide discharge peaks.

Long-term records were especially useful, as extreme events could be considered. However, the temporal resolution of precipitation data with a time step of 6 h was too low to evaluate discharge responses of the system. In conclusion, the applied approach was very useful to quan-tify flood-buffering and dampening effects of the porous-media aquifer in the karstic catchment area.

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8 Synthesis

Chapter 8

Synthesis

8.1 Conclusions

Karst aquifer systems are characterized by highly heterogeneous drainage that makes it diffi-cult to assess the groundwater resource. Sustainable management of karst water resources re-quires a detailed understanding of these complex aquifer systems. The identification of hydrau-lically important drainage structures and the quantification of related transit-times are crucial parameters that provide reliable predictions about availability, storage capability and vulnera-bility of the water resource. Basic information about the geological and hydrological setting and a review of available data are necessary to develop an initial conceptual model of the sys-tem. Depending on the hydrogeological setting and the main research question, suitable meth-ods of study need to be selected and adjusted if necessary. Especially in karst hydrogeology, special methods are required to investigate these heterogeneous aquifers. In this thesis, a com-bination of artificial tracer tests, natural tracer analyses, and discharge analyses was applied to assess drainage structures and related transit-time distributions of three different karst aquifer systems: a conduit-dominated aquifer, a fissured karst system, and a karstic catchment area influenced by a hydraulic linkage between a karst and porous-media drainage system. The re-sults are an important contribution to the general understanding of karst hydrogeology and the scope of application of the different methods used.

With respect to the research questions of this thesis (section 1.2), results indicate that:

• In the conduit-dominated and in the fissured karst system, flow paths are linked to geo-logic weak zones. Groundwater flow occurs transversely to the dip of the fold axis drain-ing to the deepest outlet of the system. Cross-formational flow was observed in both karst systems. The well-developed drainage network has a hierarchical structure.

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• The conduit-dominated karst system is characterized by short transit times of a few days from the land surface to the discharging spring, which is highly dependent on flow con-ditions. The highest flow velocities were associated with epiphreatic cave passages, while flow velocities in the phreatic cave passage were an order of magnitude lower.

Low retention properties result in high vulnerability with respect to water quality.

• In the fissured karst system, a range of transit times occurred. Short transit times of a few days were associated with well-developed karst conduits, which were highly de-pendent on flow conditions. Intermediate transit times of a few months occurred in the well-drained fissured network, and long transit times > 1 year were estimated for the poorly drained fissured network. Because of the high proportion of short and interme-diate transit times, the karst water resources are highly vulnerable with respect to water quality and quantity.

• In the fissured karst system drainage through a thick unsaturated zone results in strong interaction between karst conduits and the fissured system. Storage is attributable to conduit-matrix exchange as a result of gradient inversion. In the conduit-dominated sys-tem, diverse effects of the unsaturated zone occur depending on the degree of karstifi-cation.

• Alluvial/rockfall aquifers can play an important role as natural retention zones in karstic catchment areas. Because of strong interaction between surface flow and underground drainage, alluvial/rockfall aquifers can dampen and delay flood waves after high pre-cipitation events.

Additionally, based on the different methods and approaches used, results indicate that:

• In-cave dye tracing and monitoring can provide detailed knowledge of internal drainage structures and demonstrates the dendritic structure of karst conduits. Detailed infor-mation on flow velocities and transport parameters can be achieved for individual cave passages.

• The combined use of artificial and natural tracers was crucial to assess dominant flow compartments in heterogeneous karst drainage systems. In combination with discharge

129 characteristics of springs these methods provided specific information about karst water resources and vulnerability.

• The development of a hydrogeologic model, supported by a combination of tracer tests and discharge analysis, provided unique insights into the role of a porous-media aquifer in a karstic catchment. Parameters such as flow velocities, discharge ratios, lag times, i.e., the use of an impulse-response function, and recession coefficients were particu-larly useful to describe the hydrogeologic system and develop the conceptual model.

• The methods and approach employed are suitable in areas that are difficult to access.