Contribution of time tracers (Mg 2+ , TOC, δ 13 C TDIC , NO 3 − ) to understand the role of the unsaturated zone: A case study-Karst aquifers in the Doubs valley, eastern France

Abstract : [1] Time tracers (NO 3 À , TOC, d 13 C TDIC , Mg 2+) have been used to define the hydrodynamic behavior of a karst system: high values in NO 3 À and TOC reflect rapid infiltration and consequently a short residence time within the aquifer, whereas enriched d 13 C TDIC and high Mg 2+ 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 d 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.