Carbon 13 of TDIC to quantify the role of the unsaturated zone: the example of the Vaucluse karst systems (Southeastern France)
Introduction
The unsaturated zone (UZ) is one of the most critical features of the hydrological cycle (Williams, 1983). Aquifer's UZ has been intensively studied during the last decades. Although mass transfer throughout porous media in UZ has been quantitatively depicted and mathematical models have been currently applied (Vachaud and Chen, 2002; Sung et al., 2002), the mass movement of water and aqueous tracers through the UZ in fractured rocks are still approached from a qualitative standpoint (Martin and Dean, 2001; Lee and Krothe, 2001).
Hydrologists generally neglect water movement through the UZ. They prefer to reduce the UZ into a simple ‘black-box’, where mass transfer (MT) and pressure transfer (PT) take place simultaneously, without giving a physically and chemically consistent explanation (Dassargues, 1998).
Chemical and isotopic characteristics of water are inherited through infiltration and are acquired by groundwater throughout the UZ. The knowledge of groundwater age for the evaluation of hydrodynamic parameters requires an explanation of the TDIC origin (Eichinger, 1987; Clark and Fritz, 1997; Emblanch et al., 1998a). In some karst areas like the Vaucluse system, groundwater reaches chemical equilibrium in at least 48 h (Lastennet et al., 1995). For this reason it becomes important to monitor and analyse the water–rock interactions during infiltration and to follow chemical and isotopic evolution of water during underground flow.
Although the circulation in the SZ homogenises the signature of the underground components, for a correct water resource management it is necessary to separate the different inputs of the Zones to the total discharge. The Fontaine de Vaucluse, which hydrodynamic system has been studied since the sixties (Margrita et al., 1970), offers the opportunity to quantify the contributions of the UZ to the karstic water system.
Section snippets
Geology
The Fontaine de Vaucluse (FdV) is located in the South-Eastern France, about 30 km eastward of Avignon (Fig. 1). It is the only discharge point of a karst system which is peculiar for the surface of its recharge area (1130 km2) and famous for its considerable mean flow rate of 20 m3 s−1 (Table 1). More than 98% of the FdV water discharge is supplied by precipitations infiltrated in an area devoid of hydrological surface network and mostly covered by a natural Mediterranean vegetation (garrigue).
Tracers
The flood hydrograph separation implies that the tracer is conserved within the systems (Calmels, 1985; Dzikowski, 1995; Kosakowski et al., 2001). 2H and 18O are excellent tracers because they are constitutive parts of the water molecule. Moreover they are completely useful when the participating components have a highly contrasting isotopic composition (Lakey and Krothen, 1996).
The problem of the mass conservation arises when the natural or artificial tracers are superimposed to the H2O mass.
Validity of the method
As mentioned earlier, the most important assumption is that the application of TDIC isotopic composition in hydrograph separation requires a simple mixing rate of water without any rearrangement of the carbon dissolved species. Indeed, in the FdV case, both extreme poles have their dissolved carbon close to the equilibrium with the soil CO2. Moreover, Lastennet et al. (1995) noticed a rather good homogeneity of the chemical and isotopic tracers, depicted by the Vaucluse karst system, because of
Discussion
The direct participation of the UZ to the whole water discharged yearly is 32% (Table 3). This percentage indicates the volume of water not stored within the SZ. For the winter 1995–1996, the mean participation of the UZ to the FdV discharge is 47%. Therefore, if we split the discharge during the wet period (Table 3), the participation of the UZ does not exceed 40% in December 1995, while it rises to 49% in January and February, when the discharge increases up to 70 m3 s−1 (Fig. 5). Meanwhile
Conclusions and future prospect
Oxygen 18 shows contrasting information with respect to carbon 13 in separating spring discharge. The apparent disagreement is explained by the fact that the water molecules and the TDIC display two different transfer modalities within the aquifer. At the spring, oxygen 18 traces the outflow of water infiltrated during a given meteoric event, which is chemically and isotopically well defined.
Conversely, carbon 13 traces the transit of a dissolved carbon originated by the interaction (that is
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