Process-based characterisation of flow and transport in karst aquifers at catchment scale

The main objective of the present dissertation is the development of methods for quantification of flow and transport processes in karst aquifers. It deals with both detailed parameter studies and the evaluation of collected field data using mathematical models. Due to the presence of large-scale heterogeneities (karst conduits) in the hydraulic parameter field of karst aquifers, regulation and sustainable management of karst water resources require specific methods to characterise these systems on catchment scale. Evaluation of spring responses provides an appropriate method to obtain integral hydraulic and geometric parameters of karst aquifers but is associated with some uncertainties because of the unknown temporal distribution of recharge in these systems. This work provides a time-continuous numerical approach for the estimation of inflow into the conduit system of karst aquifers. It is demonstrated that this function can be used to determine the temporal distribution of direct recharge into the conduit system. The presented approach employs the first time derivative of a spring hydrograph together with the recession coefficient of the conduit system. The first time derivative of a karst spring hydrograph reflects the temporal change of inflow to and outflow from the conduit system. The recession coefficient controls the velocity of transmission of a hydraulic signal. The methodology has been applied to quantify direct recharge into the conduit system of the Gallusquelle catchment (Swabian Alb, Germany) after a rainstorm event. The result is in agreement with information obtained from an independent isotope study reported in literature. In a further example, the introduced approach has been applied to a recharge event initiated by snowmelt. A clearly diurnal cycle of inflow into the conduit system was calculated and is attributed to a diurnal cycle of the melting process resulting from variations of air temperature (day/night). The recession coefficient of the conduit system was obtained by spring hydrograph analysis. It corresponds to the reciprocal of the mean tracer travel time determined from an artificial tracer test covering the extent of the catchment. The calculated inflow into the conduit system and interpretation of temporal distribution of recharge are therefore based on a plausible and measurable parameter. The second part of this study deals with the quantification of reactive transport of artificial tracers and the determination of geometrical conduit parameters from multitracer tests. Therefore a large-scale multitracer test was performed and a strategy for calibration of reactive transport parameters developed applying the two-region nonequilibrium model CXTFIT2.1. The ambiguity of the reactive model was reduced by the introduction of specific ranges in which reactive calibration parameters have to fit in. The model succeeds in calibrating conservative (with uranine) and reactive transport parameters (with sulforhodamine G), and verifies that tracer/rock interactions preferably occur in the immobile fluid region of the conduit system that comprises, however, only a small part of the total conduit volume. Artificial multitracer tests are therefore a useful tool in characterising fast transport in karst aquifers and may play an important role for the characterisation of conduit surfaces in the future. Additionally, environmental tracers provide valuable information on slow transport processes through the fissured matrix of karst aquifers. The analyses of data from spring Gallusquelle has shown that simulated mean ages for the following tracers differ in the order: 3H >> 85Kr = SF6 > 3H/3He. The result indicates a slow percolation of water through the thick unsaturated zone. The interpretation is supported by distributive parameter modelling (FEFLOW5.3) of flow and transport in a fissured matrix block that comprises a thick unsaturated zone. The simulations show that the unsaturated zone of karst aquifers may provide an important water storage and indicate the existence of quasi-stagnant zones in the saturated porous matrix of karst aquifers. The presence of such zones in the Gallusquelle karst aquifer was verified by a large amount of radiogenic 4He in water samples from wells within the catchment area. However, the unambiguous interpretation of isotopic data requires further research, for example, to quantify the influence of diffusive transfer between the fissures and the porous matrix for the specific tracers.