Beurteilung der Grundwasser-Vulnerabilität in Karstgebieten anhand 3D geologischer und numerischer Modelle

Abstract

Groundwater is a major resource for mankind. Among groundwater resources, karst groundwater plays an important role. Approximately a quarter of the world's population depends on karst groundwater. Also in numerous regions of Switzerland, karst groundwater is indispensable for the drinking water supply. The ecological importance of karst groundwater as a prerequisite of sensitive habitats, like natural springs and spring brooks, is also increasingly recognized. However, karst aquifers are particularly vulnerable to contamination due to the often thin soil cover, concentrated infiltration and short groundwater residence times. Therefore, this thesis aimed at an improvement of the knowledge about karst specific hydrologic-hydrogeological processes and of methods for the evaluation of groundwater vulnerability. The super-ordinate goal was to contribute to the preservation or advancement of groundwater quality in karst areas. The applied methodology includes a 3D geological modeling of the subsurface on the one hand, and a numeric simulation of the flow processes, based on volumetric models (box models), on the other hand. For the numeric modeling, hydrological and meteorological data were recorded for more than one year. Based on the 3D geological model, an approach was developed for the localization of discharge and catchment areas and for the characterization of the underground flow paths in karst areas (ABG approach). From this, the structural framework for the hydrology of the area was established. At the same time, the response of a karst spring to rainfall events was simulated with the help of the numerical models. The simulations served to quantify the vulnerability and its temporal variation. New criteria were established for the evaluation of the vulnerability: the vulnerability index VI, which specifies the contributions from fast and slowly circulating flow systems to the spring discharge, and the vulnerability concentration Cv, which represents a potential contaminant load of the spring water. Apart from the evaluation of temporal aspects of vulnerability, the approach additionally allows comparing different springs quantitatively with regard to their contamination risk. Finally, the results from the 3D goelogical modeling were combined with the results from the numerical modeling and contemplated by vulnerability mapping in the catchment areas. From this, combined vulnerability maps and time series resulted as the final product. The proposed approach was tested at a field site situated in the Swiss Tabular Jura (Gempen plateau). Its application illustrated a new methodology how to accomplish a comprehensible and transparent delineation of protection zones. Several improvements of existing mapping methods were obtained: The spatial vulnerability distribution could be indicated not only for catchment areas but also for discharge areas, the temporal variation of vulnerability could be determined, and different springs could be quantitatively compared with regard to their vulnerability. The consideration of temporal aspects of vulnerability extends groundwater protection strategies by one dimension and facilitates differentiated solutions of problems when land use conflicts in the catchment areas occur. Restrictions for the land users in the protection zones, for instance, can be eased at times of low vulnerability (land use management) or the drinking water can be rejected at the spring captures at times of increased vulnerability (withdrawal management). The possibility of comparing the vulnerability of different springs quantitatively establishes an objective basis for regional planning. For example, an abandonment of particularly endangered springs can lead to new possibilities for regulations in the catchment areas. In addition, the presented approach encourages a potential revitalization of springs by evaluating springs that have a minor importance for drinking water supply.