Carbon 13 of TDIC to quantify the role of the unsaturated zone: the example of the Vaucluse karst systems (Southeastern France) C. Emblanch a, * , G.M. Zuppi b , J. Mudry c , B. Blavoux a , C. Batiot a a Laboratoire d’Hydroge ´ologie, Universite ´ d’Avignon et de Pays de Vaucluse, 33 rue Louis Pasteur, 84000 Avignon, France b Universita ` Ca’ Foscari di Venezia, Dipartimento di Scienze Ambientali, Calle Larga Santa Marta, Dorsoduro, 2137, 30123 Venezia, Italy c Departement de Ge ´oscience; De ´formation, Ecoulement, Transfert, Universite ´ de Franche Comte ´, Faculte ´ des Sciences et Techniques, 16 route de Gray, 25030 Besanc ¸on, France Received 7 March 2003; accepted 6 May 2003 Abstract The total dissolved inorganic carbon (TDIC) and 13 C TDIC have been used as chemical and isotopic tracers to evaluate the contribution of different water components discharging at the Fontaine de Vaucluse karst spring near Avignon. At the same time they have been used to separate its flood hydrograph. Waters flowing from unsaturated zone (UZ) and saturated zone (SZ) show similar concentration in TDIC. In UZ and SZ water rock interactions do not obey to the same kinetic. The mixing rate between water coming from the UZ characterised by a short residence time and water from the SZ with a longer residence time has been evaluated in the spring discharge. In a hydrodynamic system, which is rather complex as it is open to the soil CO 2 in UZ and closed to the same CO 2 in the SZ, 13 C TDIC has excellent characteristics as an environmental tracer. In order to better describe the inwardness of mass movements within the aquifer, the apparent contrasting information obtained using two different isotopes ( 18 O of water molecules and 13 C of TDIC) must be combined. 18 O informs whether the hydrodynamic system acts as piston flow (PF) or follows a well mixing model (WMM). Conversely, 13 C gives more complete information on the UZ contributes to the total discharge. q 2003 Elsevier B.V. All rights reserved. Keywords: Karst; Hydrograph separation; Unsaturated zone; Total dissolved inorganic carbon; Carbon 13; Oxygen 18 1. 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 quantitat- ively 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) Journal of Hydrology 279 (2003) 262–274 www.elsevier.com/locate/jhydrol 0022-1694/03/$ - see front matter q 2003 Elsevier B.V. All rights reserved. doi:10.1016/S0022-1694(03)00180-X * Corresponding author. Tel.: þ33-490-14-44-85; fax: þ 33-490- 14-44-89. E-mail address: christophe.emblanch@univ-avignon.fr (C. Emblanch).