Dynamic scheme of water circulation in karstic aquifers as constrained by Sr and Pb isotopes. Application to the Herault watershed, Southern France

This paper focuses on the relationship between the karst aquifers and the Herault River (southern France) as a function of hydrologic conditions. The combination of major and trace element, and Sr and Pb isotopes determined on dissolved load in karstic springs makes it possible to identify a dynamic scheme of the hydrology of the karstic area. In the headwaters part of this area, the Sourcettes Spring is fed by an interrupted stream that infiltrates at the basement-sedimentary cover contact. Internal geochemical processes are also pointed out: (1) water-rock interactions during underground circulation, and (2) water originating from different layers of the aquifer may show different chemistry depending on the hydrological conditions. The Vernede Spring, is fed by a second water supply during high flows (previously considered as unconnected). Finally, the Cent-Fonts Springs have been confirmed to be fed by the Buege interrupted stream. Pb isotopes on the dissolved and particulate loads of the river samples make it possible to discriminate the natural (local rocks) versus anthropogenic (mining district and road traffic) Pb sources. The Pb isotope investigations on karstic water make it possible to differentiate between neighboring springs presenting identical Sr isotope and element ratios, and to point out different underground circulation.