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Zwahlen, François
Nom
Zwahlen, François
Affiliation principale
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Professeur.e émérite
Email
francois.zwahlen@unine.ch
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- PublicationMétadonnées seulementPercolation and Particle Transport in the Unsaturated Zone of a Karst Aquifer(2009)
; ; Recharge and contamination of karst aquifers often occur via the unsaturated zone, but the functioning of this zone has not yet been fully understood. Therefore, irrigation and tracer experiments, along with monitoring of rainfall events, were used to examine water percolation and the transport of solutes, particles, and fecal bacteria between the land surface and a water outlet into a shallow cave. Monitored parameters included discharge, electrical conductivity, temperature, organic carbon, turbidity, particle-size distribution (PSD), fecal indicator bacteria, chloride, bromide, and uranine. Percolation following rainfall or irrigation can be subdivided into a lag phase (no response at the outlet), a piston-flow phase (release of epikarst storage water by pressure transfer), and a mixed-flow phase (increasing contribution of freshly infiltrated water), starting between 20 min and a few hours after the start of recharge event. Concerning particle and bacteria transport, results demonstrate that (1) a first turbidity signal occurs during increasing discharge due to remobilization of particles from fractures (pulse-through turbidity); (2) a second turbidity signal is caused by direct particle transfer from the soil (flow-through turbidity), often accompanied by high levels of fecal indicator bacteria, up to 17,000 Escherichia coli/100 mL; and (3) PSD allows differentiation between the two types of turbidity. A relative increase of fine particles (0.9 to 1.5 mu m) coincides with microbial contamination. These findings help quantify water storage and percolation in the epikarst and better understand contaminant transport and attenuation. The use of PSD as "early-warning parameter" for microbial contamination in karst water is confirmed. - PublicationMétadonnées seulementEpikarst storage in a karst aquifer: a conceptual model based on isotopic data, Milandre test site, Switzerland(2003)
; The Milandre test site is a karst aquifer characterized by diffuse infiltration, a well developed conduit network, and several tributaries feeding an underground river. Field data include discharge rate measurements, stable isotopes, weekly rainfall and spring-water isotope sampling, and detailed isotope sampling during three flood events. Flood sampling was carried out at several tributaries corresponding to conduit flow, vadose flow and seepage flow. Weekly sampling showed a strong buffering of the rainfall isotopic signal at the spring. This attenuation suggests an important mixing reservoir in the system. Flood events showed highly peaking hydraulic responses but buffered rain isotope responses. These results indicate that the soil and epikarst sub-systems have an important storage capacity. A conceptual model of flow and transport in the soil and epikarst zone is proposed: Soil plays an important role in mixing due to the presence of capillary water storage. Consequently dampened concentrations reach the epikarst despite a rapid hydraulic response. The epikarst acts as the storage element and distributes water as either a base flow component or a quick flow component. When recharge exceeds a given threshold, excess infiltrated water bypasses the soil and epikarst and reaches the saturated zone as fresh flow. Based on this model, the significance of phreatic storage is thought to be limited, at least in Milandre test site. Hence the saturated zone is seen mainly as a transmissive zone through its well developed conduit network. (C) 2003 Elsevier B.V. All rights reserved.