31 Mar 2022
31 Mar 2022
Status: this preprint is currently under review for the journal HESS.

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)

Leïla Serène1, Christelle Batiot-Guilhe1, Naomi Mazzilli2, Christophe Emblanch2, Milanka Babic2, Julien Dupont2, Roland Simler2, Matthieu Blanc3, and Gérard Massonnat4 Leïla Serène et al.
  • 1HSM, Univ. Montpellier, CNRS, IMT, IRD, Montpellier, France
  • 2UMR 1114 EMMAH (AU-INRAE), Université d’Avignon, 84000 Avignon, France
  • 3Independent researcher, Montpellier, France
  • 4TotalEnergies, CSTJF, Avenue Larribau, CEDEX 64018 Pau, 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.

Leïla Serène et al.

Status: final response (author comments only)

Comment types: AC – author | RC – referee | CC – community | EC – editor | CEC – chief editor | : Report abuse
  • RC1: 'Comment on hess-2022-100', Weiquan Dong, 22 Apr 2022
    • AC1: 'Reply on RC1', Leïla Serène, 23 May 2022
      • RC3: 'Reply on AC1', Weiquan Dong, 23 May 2022
  • RC2: 'Comment on hess-2022-100', Anonymous Referee #2, 28 Apr 2022
    • AC2: 'Reply on RC2', Leïla Serène, 23 May 2022
  • CC1: 'Comment on hess-2022-100', Neil Terry, 30 Apr 2022
    • AC3: 'Reply on CC1', Leïla Serène, 23 May 2022
  • AC4: 'Comment on hess-2022-100', Leïla Serène, 26 May 2022
    • RC4: 'Reply on AC4', Weiquan Dong, 26 May 2022

Leïla Serène et al.

Leïla Serène et al.


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Short summary
This work aims to develop the Transit Time index (TTi) as a natural tracer of karst groundwaters transit time, usable in the 0–6 months range. Based on the fluorescence of organic matter, TTi shows its relevance to detect a small proportion of fast infiltration water within a mix, while other natural transit time tracers provide no or less sensitive information. Comparison of the average TTi of different karst springs also provide consistent results with the expected relative transit times.