PCO2-13C: An interesting tool to characterize types of karstic aquifers

Carbon-13 is an interesting tracer in studying water transit within karst systems. It can also be used to characterize the different types of waters which participate in the karstic flow (fast and slow infiltration in the unsaturated zone and water from saturated zone). The δ13CTDIC (Total Dissolved Inorganic Carbon) at the outlet of a karstic system originates from the influence of the three Carbon-13 end-members: atmosphere, -8 per mille VPDB, carbonates, 0 ± 2 per mille VPDB, and biogenic CO2, values varying from -20 to -27 per mille VPDB for C3 pathway plants; and the aqueous carbonate species equilibria under closed or open conditions. Several systems were studied from springs of the experimental site of Vaucluse (Southeastern France): Fontaine de Vaucluse (FV), Notre-Dame des Anges (NDA), Grozeau (GRO), Font Gillarde (GIL), St-Trinit (TRI), Millet (MIL), Font d'Angiou (ANG), la Nesque (NES). The aquifers characteristics are already known thanks to several works. A new sampling campaign has recently be conducted, including: temperature, electrical conductivity, anions and cations, total organic carbon and δ13CTDIC. The different springs were sampled twice a month, during an hydrological cycle (from 1999 December to 2001 January). Observed variations of δ13CTDIC versus partial pressure of CO2 (pCO2) highlights two kinds of evolution for these systems: Evolution 1 corresponds to strong variations in pCO2 but strong variations in 13C and gathers the Font Gillarde, the Grozeau, Fontaine de Vaucluse and Notre-Dame des Anges. Each springs show strong variations for both pCO2 (mean value from 4.7. 10-3 to 1.8. 10-2 atm) and δ13CTDIC (mean value from -10.8 to -12.1 per mille). This first relation expresses the thickness increase of saturated zone. Indeed, the Font Gillarde and the Grozeau are poorly karstified systems, with a limited saturated zone, whereas Fontaine de Vaucluse and Notre-Dame des Anges, show a well-developed karstification with saturated zones of respectively 300 and 100 m thick. Saturated zone works as a closed system relative to the gaseous phase inducing high values of pCO2 because a lack of degassing processes. Moreover, carbon-13 contents of waters from this environment evolves towards to enriched values due to a long contact with the carbonate rock. Evolution 2 expresses low variations for the two parameters and includes the Font Gillarde, Millet, Font d'Angiou and St Trinit springs. This one present weak variations in pCO2 (mean value lower than 1. 10-2 atm) and δ13CTDIC (mean value from -11 to -13 per mille). These aquifers are superficial systems with different degree of karstification. There is no fast infiltration in Font Gillarde and Millet systems because of their hydrodynamic behaviours, characterized by a net difference between transfer and transit of water. Fast infiltration can be possible in the Font d'Angiou system. Then, St Trinit is a typical epikarstic aquifer, that is to say that infiltration of recent water is very fast (several hours). Evolution 2 highlights an increasing influence of infiltration zone on the aquifer behaviour. This classification of the karstic systems from the experimental site of Vaucluse corresponds to those established with others parameters and tracers of residence-time such as Mg, and work with others karstic aquifers from the Jura mountains (Eastern France) and Larzac (Southeastern France). The study of the pCO2 and δ13CTDIC variations is a relevant tool to characterize karstic systems