Résumé
Transit time can be estimated thanks to natural tracers but few of them are usa-ble in the 0-6 months range. The main purpose of this work was to analyze the potential of the ratio of heavy to light-weight organic compounds (HIX, Ohno, 2002; Zsolnay et al., 1999) as a natural tracer of short transit time (Blondel et al., 2012). Critical analysis of former studies showed that although the link between HIX and transit time seems consistent, the whole methodological approach need-ed to be consolidated. Natural organic matter fluorescence from 289 water sam-ples from 4 springs and 10 flow points located in the unsaturated zone of the Vaucluse karst system was characterized by parallel factor analysis (PARAFAC) thanks to excitation-emission matrix (EEM), thus (i) allowing the identification of main fluorescent compounds of sampled groundwater; and (ii) evidencing the in-adequacy of HIX 2D emission windows to characterize groundwater organic mat-ter. We then proposed a new humification index called Transit Time index (TTi) based on Ohno (2002) formula but using PARAFAC components of heavy and light organic matter from our samples instead of 2D windows. Finally, we evaluated TTi relevance as a transit time tracer by: (i) per-forming a detailed analysis of its dynamic on a selected spring (Millet) and (ii) comparing its mean value over karst springs of Fontaine de Vaucluse system. We showed that relative distribution of TTi over studied karst systems is consistent with expected transit times at short time scales. However TTi provides incon-sistent values in hydrosystems supplied by yearly water. After 6 months, only the refractory fraction of the fluorescent organic matter remains making the TTi un-usable. Principal component analysis (PCA) of hydrochemical parameters put in relief two different poles of TTi components according to classical transit time tracers. It has also led to characterize a small proportion of fast infiltration while other natural tracers of transit time provide less or no information. TTi appears as a relevant tracer of transit time in the 0-6 months range where existing tracers fail.