Contribution of time tracers (Mg2+, TOC, δ13CTDIC, NO3-) to understand the role of the unsaturated zone:: A case study -: Karst aquifers in the Doubs valley, eastern France -: art. no. 1322

Time tracers (NO3-, TOC, delta(13)C(TDIC), Mg2+) have been used to define the hydrodynamic behavior of a karst system: high values in NO3- and TOC reflect rapid infiltration and consequently a short residence time within the aquifer, whereas enriched delta(13)C(TDIC) and high Mg2+ are expected for "old water''. 9 Springs and 5 boreholes have been sampled during three field campaigns in the Doubs valley karst aquifer: low water, flood and recession periods. A clear differentiation can be highlighted between boreholes, characterized by a long residence time, and springs that show a rapid infiltration. Considering only the springs values, it appears that TOC and delta(13)C(TDIC) contents can easily be correlated to the sampling period. We show then the contribution of the unsaturated zone to the discharge during the low-water period, and the existence of reserves that seem badly connected to the drainage network, and that contribute poorly to the minimal flow.