Multipeaked breakthrough curves in karstic rivers:effects of a diffluence-confluence system

In karstic environments, it is not unusual for an underground river to split into two or more streams (diffluence) and merge back together downstream (confluence). This kind of behavior can generate multipeaked breakthrough curves (BTCs) in dye tracing at a sampling site located downstream of the confluence(s). It is also possible that such a phenomenon is difficult to highlight with dye tracing if the tracer clouds coming from the different streams reach the sampling locations at the same time. In this study, an attempt at quantifying the importance of different criteria in the occurrence of a multipeaked BTC is done by performing a dye tracing campaign in a two-tributaries diffluence-confluence (DC) system and using a one-dimensional solute transport model. The results from both field data and the solute transport model suggest that a double-peaked BTC occurs downstream of a DC system if the following conditions are met: (1) the injection is done close enough to the diffluence, (2) the sampling point is located not too far from the confluence, and (3) the two (or more) streams have sufficiently contrasted travel times from the diffluence to the confluence. The paper illustrates that, even if a diffluence occurs in a karstic river, multipeaked BTCs are not necessarily observed downstream of the confluence if these three conditions are not met. Therefore, characterizing a DC system using dye tracing is a real challenge. This could explain why publications that report studies involving multipeaked BTCs are quite rare.</p

Assessment of groundwater recharge processes through karst vadose zone by cave percolation monitoring

peer reviewe

Hydrogeological Dynamic Variability in the Lomme Karst System (Belgium) as Evidenced by Tracer Tests Results (KARAG Project)

Paleozoic carbonate aquifers represent major groundwater resources in Belgium. Karstification processes affect most of them and Belgium counts many hydrologically active karst networks. Given the intrinsic vulnerability of such geological objects, comprehensive studies are required in order to improve their protection and management. The KARAG project (Karst Aquifer ReseArch by Geophysic, 2013-2017) aims to identify the specific dynamic of karst aquifers by using geophysical and hydrogeological tools. This research is funded by the Belgium National Fund for Scientific Research (FNRS) and conducted by the University of Namur, University of Mons and the Royal Observatory of Belgium. The LKS – Lomme Karst System (Rochefort, southern Belgium) was chosen as it is a major Belgian karst system (10 km long) in the Givetian carbonate aquifer (Middle Devonian). The system is formed by two parallel components: the surface system (the Lomme River) and a complex underground system (multiple sinkholes with one main resurgence). Based on this layout, it is possible to study the aquifer dynamic and its relationship with the surface river. A high resolution monitoring has been installed since July 2013 in order to follow the system dynamic during several hydrogeological cycles. Multi-tracing experiments with different injections and monitoring points highlight the complexity of underground flow dynamics. Investigations enlightened the connectivity between monitoring points and how dependent of the hydrological conditions were these connections. The breakthrough curves analysis allows to characterize the hydrodynamic behavior of the underground flows within the aquifer. Modeling will be necessary to link hydrological and tracer tests data in order to build a detailed conceptual model for this karst system. This model will also be used to interpret geophysical data (ERT, gravimetry) collected in order to study the unsaturated and epikarst zones