Geochemistry and water dynamics of a medium-sized watershed: the Hérault, southern France: 1. Organisation of the different water reservoirs as constrained by Sr isotopes, major, and trace elements

The aim of this study of the Hérault watershed is to constrain the river–groundwater relations with a global geochemical investigation: major and trace elements, radiogenic isotopes (Sr, Pb) and stable isotopes (oxygen, deuterium). Among the six sampling campaigns in high and low flows, this paper focuses on the first one (March 1995) sampled during a low flow period for major and trace elements and Sr isotopes on both dissolved and particulate loads. A companion paper will focus on the river–karst relations in a dynamical scheme over a complete hydrological cycle. The objective of this paper is to constrain the spatial organisation of the different water reservoirs, to show the relations between surficial and groundwaters and to assess the mechanical and chemical weathering in this low-flow period. The Hérault main stream successively drains a Palaeozoic basement, a karstified Mesozoic cover and a Tertiary and Quaternary alluvial plain, the different tributaries also drain these lithologies. This watershed is also impacted by ancient mining, and agricultural activities especially in the southern part. Major element concentrations generally reflect the drained lithologies. The chemistry of the Hérault main stream is mainly controlled by silicate and carbonate endmembers, whereas the karstic springs show clearly a mixture of limestones and dolomitic limestones. Major elements point out some specific characteristics of some karstic springs, and Sr isotopes are used to check previous underground circulation hypotheses and sometimes reveal strong connections with overlying surface waters. Mixing phenomena between the Hérault river and its tributaries can be quantified in the 87 Sr / 86 Sr vs. 87 Rb / 86 Sr diagram based on a simple two-component scheme and agree within 10% with the real discharge measurements. As in large basins, the suspended matter presents more variable and more radiogenic 87 Sr / 86 Sr than the dissolved load, due to the more important contribution of silicate particles which are less soluble than carbonate. Dissolved Ca fluxes were estimated to be 36, 51 and 38 kg km −2 day −1 in the Palaeozoic basement, the Mesozoic cover and the alluvial plain, respectively. This implies a chemical erosion estimated to be 20 μm/year in the Palaeozoic basement and 58 μm/year in the Mesozoic cover, similar to values calculated for large basins.