Transit Time index (TTi) as an adaptation of humification index to illustrate transit time differences in karst hydrosystems. Application to the karst springs of Fontaine de Vaucluse system (Southeastern France)

Abstract. Transit time can be estimated thanks to natural tracers but few of them are usable in the 0–6 months range. The main purpose of this work is 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 shows that although the link between HIX and transit time seems consistent, the whole methodological approach needs to be consolidated. Natural organic matter fluorescence from 289 water samples from 4 springs and 10 flow points located in the unsaturated zone of the Vaucluse karst system is 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 inadequacy of HIX 2D emission windows to characterize groundwater organic matter. We then propose 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 evaluate TTi relevance as a transit time tracer by: (i) performing a detailed analysis of its dynamics on a selected spring (Millet) and (ii) comparing its mean value over karst springs of Fontaine de Vaucluse system. Principal component analysis (PCA) of TTi and other hydrochemical parameters monitored at Millet put in relief the different ranges of transit time associated with the different organic matter compounds. PCA results also provide evidence that TTi can detect a small proportion of fast infiltration water within a mix, while other natural tracers of transit time provide no or less sensitive information. TTi distributions at monitored karst springs are consistent with relative transit times expected for the small-scale, short average transit times systems. TTi thus appears as a relevant tracer of transit time in the 0–6 months range where existing tracers fail, even if the information on transit time is only qualitative at this stage.