Characterisation of Karst Systems by Simulating Aquifer Genesis and Spring Responses : Model Development and Application to Gypsum Karst

Abstract

A process-based numerical modelling tool is developed, designed to support the charcterisation of karst aquifers using two complementary approaches: Firstly, the simulation of solutional conduit enlargement, which aims at predicting aquifer properties by forward modelling of karst genesis; secondly, the simulation of heat and solute transport, which aims at inferring aquifer properties from short-term karst spring responses. Karst genesis modelling is applied to a conceptual setting based on field observations from the Western Ukraine, where gypsum layers are typically supplied by artesian flow of aggressive water from insoluble aquifers underneath. Processes and parameters, controlling solutional enlargement of single conduits under artesian conditions, are identified in detailed sensitivity analyses. The development of conduit networks is examined in parameter studies, suggesting that the evolution of maze caves is predetermined by structural preferences such as laterally extended fissure networks beneath a horizon less prone to karstification. Short-term karst spring response after recharge events is firstly examined in parameter studies by forward modelling. The numerical simulations reveal that different controlling processes of heat and solute transport account for the different behaviour of water temperature and solute concentration frequently observed at karst springs. In order to test the feasibility of the inverse approach, the model is applied to a field site in Southern Germany (Urenbrunnen, Vöhringen). Several models, which reproduce the results of a combined tracer and recharge test, are calibrated to spring discharges and solute concentrations measured after a recharge event. In order to validate the calibrated models, the measured spring water temperatures are simulated by heat transport modelling. The model application yields information on aquifer properties as well as on flow and transport processes at the field site.