Groundwater modelling in karst terrains

Groundwater modelling has become a standard technique regularly applied, e.g., for engineering tasks and water management purposes. To date, most practical model applications deal with groundwater in porous media. Specific modelling tools were developed to account for the peculiarities of karst aquifers. Yet these state-of-the art models are frequently applied for basic research into karst evolution as well as flow and transport phenomena in karst terrains but rarely used for practical purposes. Thus, the overall objective of the proposed project is to narrow the gap between basic research into karst modelling and the needs of practitioners working in karst areas. To this end, the practical applicability of state-of-the art models will be evaluated based on benchmark tests and model applications at well-investigated test sites. The results from these investigations will be summarised in general guidelines for the practical use of groundwater models in karst terrains. These guidelines are sought to advise appropriate methods for conceptual model development, numerical model design, and model calibration depending on the purpose of the model application. The project builds on earlier basic research into the characterisation and modelling of karst aquifers as well as into the hydrogeology of the selected test sites. The preliminary conceptual models derived from earlier research will be further developed and adapted for the purpose this project. The application of the numerical models will be focused on the simulation of hydraulic aquifer responses to recharge events using both single-continuum and hybrid models. In addition, physicochemical responses and tracer transport will be considered, in particular, to reduce the ambiguity of the model calibration. It is anticipated that the model application reveals the need for improvements on existing approaches within each of the modelling steps: Multiple methods for the aquifer characterisation need to be combined and potentially adapted for the development of appropriate conceptual models of the test sites; numerical state-of-the art models may have to be modified to allow the technical implementation of these conceptual models; model calibration may require the use of innovative parameter optimization techniques. Experiences and improvements concerning these individual aspects will be integrated in an overall conceptual framework addressing the practical use of state-of-the-art karst models.

Groundwater models are frequently applied for practical purposes, for instance, with regard to the management and protection of water resources. To date, practical applications of such models mainly address groundwater in unconsolidated sediments such as sand or gravel, where the water moves slowly through the narrow pore spaces within the grain structure. In contrast, the dissolution of karst rocks (such as limestone) may cause a widening of fractures, thus creating much larger cavities with diameters ranging from centimetres to metres. The rapid and often turbulent flow in such solution conduits is generally not adequately considered by the prevalent groundwater models. Within this project, therefore, more recent modelling approaches accounting for the peculiarities of karst areas were tested and further developed. To this end, benchmark tests were performed and various modelling approaches were applied to well-investigated test sites. It was found that the occurrence of turbulent flow may have significant effects on the transient discharge behaviour of karst springs. The representation of these processes in the models employed was found to be inadequate, in particular, with regard to the applicability for the test sites. For this reason, a more robust and easily applicable approach accounting for non-linear (turbulent) flow was developed and implemented into the widely-used groundwater model MODFLOW. In addition, the example applications resulting from this project demonstrate how groundwater models can support the characterization of the properties and functioning of karst aquifers and, in particular, the delineation of spring catchments, thus contributing to practical issues such as the designation of protected areas.