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International audienceFor about ten years, environmental tracing development using Dissolved Organic Matter (DOM) has been the subject of several studies. Particularly, the use of characterization techniques, like fluorescence Emission-Excitation Matrices (EEM), has enabled identification and monitoring of DOM sources within mainland or marine hydrosystems. Hydrogeologists have already shown the significance of Total Organic Carbon (TOC) used as fast seepage tracer in karstic aquifers. The aim of this study consists in using DOM fluorescence signals to develop a transit time quantitative tracer in heterogeneous hydrosystems. The Low-Noise Underground Laboratory of 'Rustrel - Pays d'Apt' (France) offers a special access to different kinds of unstructured karstic flows, cutting randomly through the fault network of 'Fontaine de Vaucluse' vadose zone. The hydrochemical monitoring of these flows since 2002 has allowed their behaviour to be well known, and therefore to calibrate a relationship between a fluorescence index (Humification IndeX) and their mean transit time. Finally, this relationship has been tested on two springs of 'Vaucluse' plateaus, giving good transit time estimations for hydrosystems which do not present mixture between recent and pluriannual waters

Evaluating transit time by using delta(18)O seasonal variation is often difficult in a Mediterranean context due to the erratic rainfall signature added to the complexity of flow mixing. The present study aims to show that using organic matter can improve interpretations of the delta(18)O signal. The natural fluorescence of organic compounds and dissolved organic carbon (DOC) data were recorded in the underground low-noise laboratory, located within the catchment area of the Fontaine de Vaucluse karstic system, over a four-year period. By combining both total fluorescence and DOC, a systematic seasonal variation is observed only due to soil-water interaction. Therefore, when the recharge rate is enough at the time of the season concerned, a new specific seasonal tracer, independent of rainfall signature, is available. Besides, the DOC concentration is a tracer of rapid infiltration with short transit time associated with high DOC concentration, while long transit time waters are characterised by a low DOC concentration. Then this can also shed light on such a mixing, recent/old waters. A more sensitive interpretation of delta(18)O variations is then possible: if the rainfall amount and delta(18)O follow a seasonal trend, both tracers may be used in the same way; if the recharge is discontinuous due to discontinuous rainfall regime, delta(18)O tracing alone is usable; in case of erratic or smooth rainfall signature during the homogeneous rainy period, fluorescent organic matter tracing alone is then usable.

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