Abstract
We assess the errors produced by considering temperature as a conservative tracer in fluviokarst studies. Heat transfer that occurs between karstic Conduit System (CS) and Porous Fractured Matrix (PFM) is the reason why one should be careful in making this assumption without caution. We consider the karstic aquifer as an Open Thermodynamic System (OTS), which boundaries are permeable to thermal energy and water. The first principle of thermodynamics allows considering the enthalpy balance between the input and output flows. Combined with a continuity equation this leads to a two-equation system involving flows and temperatures. Steady conditions are approached during the recession period or during particular phases of pumping test experiments. After a theoretical study of the error induced by the conservative assumption in karst, we have applied the method to revisit the data collected during a complete campaign of pumping test. The method, restricted to selected data allowed retrieving values of base flow, mixing of flow, intrusions of streams, and aquifer answer to drawdown. The applicability of the method has been assessed in terms of propagation of the temporal fluctuations trough the solving but also in terms of conservative assumption itself. Our results allow retrieving the main hydrological properties of the karst as observed on field (timed volumetric samplings, geochemical analyses, step pumping test and allogenic intrusion of streams). This consistency argues in favor of the applicability of the conservative temperature method to investigating fluviokarst systems under controlled conditions.
Highlights
Introduction and Presentation of CentFonts FluviokarstThis paper addresses the issue of using the water temperature as a conservative tracer for karstic functioning studies
An Open Thermodynamic System (OTS) does not require a precise knowledge of the locations and shapes of its boundaries to study the balance of the fluxes that enter or quit its Control Volume (CV)
The heat and matter exchanges occurring through the karstic boundaries impose a consideration of the Open Thermodynamic System (OTS)
Summary
This paper addresses the issue of using the water temperature as a conservative tracer for karstic functioning studies. Several works [1,2] study the water exchanges between underground flows and surface stream. Many situations as recession in karst undergo naturally damping of the temporal fluctuations both for temperatures and flows These preliminary considerations justify the interest to study the karst systems as an Open. Buèges stream flows permanently on Triassic low permeability outcrops it disappears à few kilometers after Saint-Jean de Buèges where the course crosses a Bathonian, calcareous dolomite swallow zone After this area, the Buèges stream course follows a valley that remains dry most of the year except after major rainfalls. The Cent-Fonts resurgence flows into Hérault River through a shallow network of springs that gush out a few tens of centimeters above the base level. A spring arises directly through the bottom of the Hérault River bed [20]
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