GEOCHEMISTRY AND WATER DYNAMICS OF A MEDIUM-SIZED WATERSHED - THE HERAULT, SOUTHERN FRANCE 1 - ORGANIZATION OF THE DIFFERENT WATER RESERVOIRS AS CONSTRAINED BY SR ISOTOPES, MAJOR, AND TRACE-ELEMENTS

The aim of this study of the Herault watershed is to constrain the riv er-groundwater relations with a global geochemical investigation: majo r 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 durin g a low flow period for major and trace elements and Sr isotopes on bo th dissolved and particulate loads. A companion paper will focus on th e river-karst relations in a dynamical scheme over a complete hydrolog ical cycle. The objective of this paper is to constrain the spatial or ganisation of the different water reservoirs, to show the relations be tween surficial and groundwaters and to assess the mechanical and chem ical weathering in this low-flow period. The Herault main stream succe ssively drains a Palaeozoic basement, a karstified Mesozoic cover and a Tertiary and Quaternary alluvial plain, the different tributaries al so 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 Herault main stream is mainly controlled by sil icate and carbonate endmembers, whereas the karstic springs show clear ly a mixture of limestones and dolomitic limestones. Major elements po int 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 Herault river and its tributaries can b e quantified in the Sr-87/Sr-86 vs. Rb-87/Sr-86 diagram based on a sim ple two-component scheme and agree within 10% with the real discharge measurements. As in large basins, the suspended matter presents more v ariable and more radiogenic Sr-87/Sr-86 than the dissolved load, due t o 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 c over and the alluvial plain, respectively. This implies a chemical ero sion estimated to be 20 mu m/year in the Palaeozoic basement and 58 mu m/year in the Mesozoic cover, similar to values calculated for large basins. (C) 1998 Elsevier Science B.V. All rights reserved.