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

[摘要] Transit time can be estimated thanks to natural tracers, but few ofthem are usable in the 0–6-month range. The main purpose of this work is toanalyze the potential of the ratio of heavy- to light-weight organiccompounds (the humification index (HIX); Ohno, 2002; Zsolnay et al., 1999) as a natural tracer ofshort transit time (Blondel et al., 2012). Critical analysis of formerstudies shows that although the link between HIX and transit time seemsconsistent, the whole methodological approach needs to be consolidated.Natural organic matter fluorescence from 289 groundwater samples from four springs and 10 flow points located in the unsaturated zone of the Vauclusekarst system is characterized by parallel factor analysis (PARAFAC) thanksto the excitation–emission matrix (EEM), thus (i) allowing for the identification ofmain fluorescent compounds of sampled groundwater and (ii) evidencing theinadequacy of HIX 2D emission windows to characterize groundwater organicmatter. We then propose a new humification index called the Transit Time index(TTi) based on the Ohno (2002) formula but using PARAFAC components of heavy andlight organic matter from our samples instead of 2D windows. Finally, weevaluate TTi relevance as a transit time tracer by (i) performing adetailed analysis of its dynamics on a selected spring (Millet) and (ii) comparing its mean value over karst springs of the Vaucluse karst system.Principal component analysis (PCA) of TTi and other hydrochemical parametersmonitored at Millet spring put in relief the different ranges of transittime associated with the different organic matter compounds. PCA resultsalso provide evidence that TTi can detect a small proportion of fastinfiltration water within a mix, while other natural tracers of transit timeprovide no or less sensitive information. TTi distributions at monitoredkarst springs are consistent with relative transit times expected for thesmall-scale, short average transit time systems. TTi thus appears as arelevant qualitative tracer of transit time in the 0–6-month range whereexisting tracers fail and may remain applicable, even in the case ofanthropic contamination thanks to PARAFAC modeling. Transforming it intoquantitative information is a challenging task which may be possible thanksto intensive studies of organic matter degradation kinetics in naturalwaters with the help of radiogenic isotope usage or an artificial tracer test.

[学科分类] 妇产科学