Transport von Mikroorganismen in einem Karstaquifer am Beispiel der Lützelquelle

Karst terrains cover about 25% of the earth’s surface and karst aquifers are estimated to provide about a quarter of the world’s population with potable water. Unfortunately, these important groundwater resources are very susceptible to microbial contamination especially after precipitation events due to point recharge, and reduced filtration in conduit systems. During dry weather periods, however, when slow to intermediate flow systems are dominant, only low levels of contamination occur. To minimize risk to public health when using karst waters for drinking water production, the local input distribution and the temporal occurrence of microorganisms at springs should be known. Based on this knowledge a water intake management scheme for karst spring waters can be established. To gain a better understanding of microorganism transport phenomena in karst systems, indicator microorganisms such as E. coli, enterococci, C. perfringens and heterotrophic plate count bacteria were analysed several times a day during 11 weeks at Lützel Spring in Northwestern Switzerland. In addition, artificial particle (bacteriophages H4/4, H40/1 and microspheres) and dye (uranine) tracer tests were performed. Along with the experiments discharge and precipitation were measured continuously. Three different approaches were used to evaluate particle transport processes and to characterize the karst system from the measured data: 1) comparison of microorganism and turbidity breakthrough curves (BTC) and changes in spring discharge after precipitation events, 2) fitting of transport parameters and calculating their possible relationship to discharge and precipitation and 3) simulation of water flow and particle transport using a sequence of box models. The results clearly show that high concentrations of indicator microorganisms and pathogens occur at Lützel Spring about 35 - 116 hours after the beginning of a precipitation event. The increase in microorganism concentrations is combined with an increase in spring discharge and turbidity. Spring discharge due to hydraulic conditions in the aquifer, always reacts the first on precipitation events, whereas turbidity raises at the same time or after the microorganisms. Furthermore, the shape of the microorganism and turbidity BTC reveal only one input location for fast water flow and contaminant transport in the catchment area of Lützel Spring what was confirmed by the artificial tracer tests performed. Particle transport from that input location to Lützel Spring at a distance of 1250 m can be described by a 1-D advection-dispersion model. The transport parameters derived show equivalent transport behaviour of almost all indicators. However, there is a variance in flow velocity and dispersion from one sub-precipitation event to another which can only partly be explained by relationships with discharge and precipitation. The derived dispersivities as well show a great variance during different hydrologic conditions, most probably because of variations in water content of the unsaturated zone and the interplay of slow, fast and intermediate flow. Box model simulations confirmed particle transport in a single channel system. Transport is induced by infiltrating precipitation which causes mechanical, chemical or biological desorption in the soil. During transport through the karst channel network only marginal adsorption and/or desorption occurs. A further transport process observed is size exclusion of particles, which results in higher mean transport velocity of particles compared to solutes. Additionally, the model simulations reveal slow, intermediate and fast flow systems that can be attributed to different geologic formations. An important feature in intermediate flow, substantially contributing to peak flow, is the overflow, a hydraulic connection from the soil surface or epikarst zone to the spring. However, particle transport is restricted to fast flow system. The field experiments and model simulations performed, allow to characterize the karst aquifer studied and to forecast high concentrations of microorganisms in Lützel Spring water. Consequently, a water intake management scheme for Lützel Spring can be established what substantially contributes to save drinking water production.