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Application of geochemical tracers is recent in Hydrology, but they more and more prove to bring important information about water movements and paths in surface processes as well as about the origin(s) and mixing proportions of their transported loads (dissolved and particulate). Whereas geochemistry clearly shows mixing phenomena on large basins (Negrel et al., 1993), the "geochemical decomposition" is more difficult on short-time and small-basin scales, particularly when the area is subject to human activities. Major, trace elements and Sr and Pb isotopes (Luck and Benothman, 1996) in dissolved and particulate (SPM, suspended particulate matter) phases are used to constrain the observed phenomena.
The Vène watershed (ª 70 km2), located on the Mediterranean coast, is composed of jurassic karstified limestones overlain in the middle by miocene marls.It is caracterized by a number of human activities (roads, vineyards, harbour, industries). Water samples were collected in sept. 1994 during the first flood during 3 days on an hourly basis at the estuary of the river and occasionally at 2 karstic resurgences. Samples were stored in acid-cleaned PP bottles and filtered under class 100 clean air a few hours after. Trace concentrations were determined by ICP-MS.
Concentrations: A first peak is observed for almost all elements, be it major or trace, interpreted as aerosol scavenging. The marine input is seen in the variation of the Na/Cl ratio. However, based on Cl correction, atmospheric correction does not exceed 10% for most elements. Depending on the elements, a regular decrease is observed (Na, K, Cl, Rb...) whereas other elements increase more or less regularly to starting values (Ca, Mg, HCO3...). Other elements such as Sr show short-time scale oscillations.
Sr isotopic compositions and 87/86Sr vs.[X]/[Y] elemental ratios: Atmospheric Cl-based correction lowers the measured values but does not affect overall variations : the values are similar to those observed over the year, but after an increase that cannot be entirely attributed to seawater influence (miocene? fertilizers?), the 87/86Sr decreases and shows with time increasing oscillations between 2 endmembers : karst-as measured by the resurgence-and miocene soils. These variations are interpreted as representing short-time oscillating mixing between karstic and surface waters at the exutory.
Samples plot between 3 main endmembers (atmosphere, karst, miocene) and show the evolution with time of the chemical signature of the outpouring waters, that can be identified to 3 main steps : 1- scavenging of aerosols;
2- leaching of miocene soils by running water; 3- increasing (with oscillations) influence of karstic water. Trace element ratios and Sr isotopes on the SPM show the various and variable origins (dust from North Africa, Miocene...) of the
particles transported.
Luck, J.-M. & Benothman, D., J. Conf. Abs. 1, (1996).
Negrel, P., Allègre, C.J., Dupré, B. & Lewin, E., Earth Planet. Sci. Lett., 59-76 (1993).