Quantification and forecasting of regional groundwater flow and transport in a karst aquifer (Gallusquelle, Malm, SW. Germany)

Martin Sauter

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Karstaquifers represent an important groundwater resource and they are highly vulnerable with respect to contamination due to fast transport through the karst system and due to the limited attenuation of contaminants in karst terraines. In order to understand such a complex system, consisting of the compartments epikarst, vadose zone and the phreatic saturated zone, and to predict the variations in concentrations of point and non-point source contaminants, numerical groundwater modelling techniques are frequently employed. This study considers two main aspects: firstly, the selection of an appropriate modelling tool for the simulation and prediction of groundwater flow and transport in a karst aquifer system, the adaptation of the model to the specific problem, the construction, testing as well as validation of the model. Secondly, the provision of data input, required for the modelling of such a complex flow system. A Double-Continuum Porous Equivalent Model was found to be the most appropriate for the study area. Each of the two flow systems of the aquifer, the fast flow, i.e. conduit system, and the slow, i.e. fissured system, is simulated with a separate continuum, and both are coupled by an exchange term. The model applies to flow from a catchment, that discharges at a spring, or where the flow into a river via alluvial deposits can be quantified. A large number of borehole logs together with information from water level fluctuations and fluvial history enabled the identification of aquifer base and zones of higher and lower hydraulic conductivities. A soil moisture balance approach together with a water balance for the canopy (accounting for interception loss) produced sufficiently accurate values for groundwater recharge. Important parameters in the recharge calculation are the field capacity the interception capacity and the rapid recharge threshold. Although the rapid recharge component as a total percentage of the annual recharge is of only minor importance, it provides recharge for periods, where ot