Isotope palaeohydrology and the prediction of long-term repository behaviour |
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| Authors: | Jean-Charles Fontes |
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| Institution: | Laboratoire d'Hydrologie et de Géochimie Isotopique, Universitéde Paris Sud, F-91405 Orsay Cedex, France. |
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| Abstract: | There are considerable, well-recognized difficulties involved in the investigation of very low permeability rocks by conventional measurements of water potentials and spatial distribution of hydrological parameters such as permeability and effective porosity. Therefore, most studies of predictive behaviour are based on models which, when unconstrained by data, are often worse than no model at all. Geochemistry and environmental isotope geochemistry are techniques well suited for hydrological investigations of potential sites of nuclear waste isolation and for constraining predictive models. Stable isotopes constitutive of the water molecule (2H and 18O) may give information on the origin of the water, especially 2H which is practically uninvolved in isotope exchange with rock-forming minerals. Variations in stable isotope contents can be related to palaeohydrological and palaeoclimatic conditions whereas 3H is indicative of recent recharge. The 13C content of dissolved carbon is related to its origin(s) and the 14C activity gives a time-scale. However, the initial carbon isotope contents can be modified by various diagenetic effects such as precipitation/dissolution of secondary calcite and oxidation of reduced carbon. The heavy isotope content of the Total Dissolved Inorganic Carbon (TDIC) cannot be interpreted without a detailed knowledge of the carbon chemistry and of the mineralogy and isotope contents of secondary carbonates. Species of Dissolved Organic Carbon (DOC), with high molecular weights (HMW) can generally be attributed to soil derived compounds. Their 14C content may be measured by AMS (Accelerator Mass Spectrometry) thus providing an estimate of the time elapsed since recharge took place. Chlorine-36 is an interesting potential tool for dating old ground waters because of its long half-life (301 kyr) and its hydrophilic behaviour. It can be also produced in situ in significant amount if the rocks are rich (some tens of ppm) in U and Th. In such cases, this time dependent build-up provides a more systematic radiochronometer of CI- residence time in the rock than the decay of cosmic input 36CI, provided no Cl- is leached from the rock. The isotopic composition of aqueous sulphur compounds depends on their origin and on their diagenetic evolution. The redox reactions of SO2-4 reduction or of S2- oxidation are marked by strong isotope effects especially when generated by bacteria. Sulphate reduction by organic matter is a potential source of dissolved inorganic carbon and may thus interfere with 14C age interpretations. After discussion of the possible age and origin of waters and solutes, some scenarios of evolution of local and regional palaeohydrological conditions are proposed. Examples are given with special emphasis on detailed results from the Fennoscanian shield (Stripa project), and from northern Switzerland. |
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