HR: 0830h
AN: V51C-0303    [PDF]
TI: Interest of 238U-234U-226Ra Disequilibrium for Tracing Groundwater Inputs Into Surface Waters: A Case Study
AU: * Chabaux, F J
EM: fchabaux@illite.u-strasbg.fr
AF: CGS-EOST Universit‚ de Strasbourg, 1 rue Blessig, Strasbourg, 67084 France
AB: Characterization of deep water inputs into surface waters requires geochemical tracers that develop different signatures in surface and ground waters. It has been suggested that U activity ratios could play such a role (e.g. Riotte et Chabaux 1999). Here we propose to test this hypothesis in the case of the Lauter stream (East France-West Germany), a stream that drains sandstone of the Bundsandstein sequence, in which several superimposed and more or less independent aquifers are developed. Major elements, trace elements, Sr isotope ratio and 238U-234U-226Ra disequilibrium have been analysed in the dissolved load of water samples collected in the Lauter stream and in the different groundwaters of the sandstone formations. The data certainly give the first convincing demonstration of the usefulness of U activity ratios in surface waters to quantify the influence of deep waters upon surface waters. The data point out that, in the specific case of the Lauter stream, more than 50% of the total dissolved load carried by the stream is supplied by deep waters. This contribution significantly modifies the 226Ra/238U activity ratio, which in turn suggests that 238U-226Ra disequilibria could also be helpful for discriminating in the global chemical fluxes carried by rivers, those representative of surface weathering processes. Overall these results highlight (1) the necessity to distinguish surface weathering fluxes from deeper ones in order to establish realistic weathering budgets at the scale of a watershed from chemical analyses of river waters and (2) the potential of 238U-234U-226Ra disequilibria to perform such distinction. Riotte et Chabaux (1999) Geochim. Cosmochim. Acta 63 1263-1275
DE: 1030 Geochemical cycles (0330)
DE: 1040 Isotopic composition/chemistry
DE: 1806 Chemistry of fresh water
DE: 1815 Erosion and sedimentation
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2003 Fall Meeting