A. Gautschi

Thermal constraints on crustal rare gas release and migration: Evidence from Alpine fluid inclusions

The Palfris marl of the Helvetic Alpine Nappes contains four distinct vein fill generations. CH4-rich gas is found in abundant fluid inclusions within these carbonate veins, while free CH4 gas has also been produced from exploratory boreholes through this formation. The stable isotope and helium, neon, and argon isotopic composition of these fluids has been determined. A constant radiogenic 40Ar concentration of 1.25 ± 0.13 (1σ) ppm in these differently sited fluids requires an intimate association between the 40Arrad source and the hydrocarbon phase. This can only be reasonably explained if the 40Arrad was input into the hydrocarbon phase during hydrocarbon generation, migration, or storage prior to entrapment in the fluid inclusions. Stable isotope results constrain the maturity of hydrocarbon production, while fluid inclusion formation pressures and temperatures record values of up to 2.5 kbars and 250°C. These values place limits on the range of thermal conditions in which the hydrocarbon/40Arrad relationship was established. All fluids within inclusions also contain radiogenic 4He40Ar values at predicted crustal production ratios. These observations provide the first evidence that both 4Herad and 40Arrad can be quantitatively released on a regional scale bounded by the thermal conditions required to produce the hydrocarbon phase and the conditions under which the fluid inclusions were formed (T = 190–250°C). These results require that negligible quantities of excess 40Arrad, decoupled from 4Herad, have been released into this system. Given the wide array of mechanisms which can potentially cause decoupling of these two species, this result provides an important constraint on the role of these processes within the sedimentary fluid regime. In contrast, the free borehole gas contains excess radiogenic 4He and 21Ne, relative to 40Arrad, in proportions which can be accounted for by local production and subsequent diffusion from the surrounding marl. The latter pattern is consistent with rare gas migration in lower temperature environments. A conceptual model which considers both diffusional and metamorphic release of helium and argon, and the ability of the surrounding fluid regime to transport the rare gases from their respective mineral production sites, is consistent with both these results and data from regional rare gas studies.

Comparison between in situ and laboratory diffusion studies of HTO and halides in Opalinus Clay from the Mont terri

Summary A long-term single-borehole diffusion experiment (DI) using tritiated water (HTO) and stable iodide (127I-) was carried out In the Opalinus Clay of the Mont Terri Underground Rock Laboratory (URL). Diffusion coefficients DL and accessible porosity for HTO, 36Cl- and 125I- were also measured on centimetric Opalinus clay samples using the through diffusion technique. The evolution of tritium and iodide concentration in the injection system over time and in situ profiles were interpreted with a 3-D numerical simulation. A detailed analysis of the results pointed out the effect of a disturbed zone around the borehole with higher diffusion coefficients. The best estimate values for HTO and iodide in the undisturbed rock are DL = 5×10-11 m2/s and DL = 1.5×10-11 m2/s respectively. For the laboratory tests, DL values for HTO are in the range of 5×10-11 m2/s to 8.5×10-11 m2/s. For 125I- and 36Cl- the measured values are DL=νmber1.4×10-11 and DL=1.6×10-11 m2/s respectively. All HTO results obtained with a through diffusion technique are within the same range as those obtained in the in situ tests. The DL values obtained in diffusion cells with 125I- and 36Cl- and the value drawn from the interpretation of stable 127I- concentration profiles from the in situ tests are very close. In fact, some significant uncertainties could be identified (i.e. a likely chemical retention of iodide on argillites, effect of the disturbed zone).

Chemical evolution of waters during long term interaction with granitic rocks in northern Switzerland

Abstract As part of an assessment of crystalline rock as a potential host for a nuclear waste repository, water samples were collected from more than 50 locations from the crystalline basement where it is under sedimentary cover in Northern Switzerland and where it is at the surface in the Black Forest. These samples describe the changing chemistry of water in an extended flow system from dilute recharge waters far from chemical equilibrium with its host rock to saline waters at temperatures of 50 to 100°C with residence times far in excess of 105 a that have reached chemical equilibrium with their host rock. This unique set of samples allows an analysis of the chemical evolution of granitic waters from surficial waters far from equilibrium to almost equilibrated waters. Mobile ions, rare gases and isotopic data are used to estimate the extent of reaction between waters and their host rock. The evolution of controlled elements is interpreted as a function of this extent of reaction. Silica contents correspond to approximate equilibrium even in the recharge waters. The relative concentrations of Ca and especially Mg are significantly lower in the borehole waters than in recharge waters indicating a trend towards equilibrium. The Na/K ratios correspond to equilibrium only in the most evolved, older, waters which are shown to be at full equilibrium.

Derivation and application of a geologic dataset for flow modelling by discrete fracture networks in low-permeability argillaceous rocks

Abstract Argillaceous rock formations are targets of exploration for radioactive waste disposal sites in several countries. Groundwater flow in most indurated argillaceous rocks is very limited and occurs (if at all) mainly in brittle discontinuities, such as faults. On the basis of surface observations, core logging and hydraulic measurements in boreholes penetrating an argillaceous marl formation in the Swiss Alps, the relationships between internal fault architecture, larger-scale arrangement of the fault network and fault transmissivity are explored. Only a fraction of all faults observed in the cores correlates with water inflow points into the boreholes, and this is taken as evidence of variable transmissivity within each fault. Such flow channeling is also supported by geologic evidence. Stochastic discrete fracture network models are used for the upscaling of measured fault transmissivities, namely for the calculation of effective hydraulic conductivities ( K eff ) of model cubes with lengths of side of 50–500 m. Input data to these models include size, spacing, orientation, heterogeneity (flow channeling) and transmissivity of the faults. Fault size is not well known, but a sensitivity analysis shows that even the extreme assumption of infinite size yields K eff only less than 1 order of magnitude higher when compared to the base case. The effect of different arrangements of flow channels within each fault is also explored but has no appreciable effect on K eff . It is concluded that calculated values for K eff are robust because those input parameters that are least adequately known have only limited effects on the model results.

The development of a safety assessment model of the geosphere for a repository sited in the crystalline basement of northern Switzerland

Abstract A modelling study of groundwater flow and radionuclide transport in the geosphere surrounding a repository for vitrified high-level waste sited in the crystalline basement of northern Switzerland has been carried out as part of the recently completed Kristallin-I project. The study derives information from an analysis of seven deep boreholes, along with seismic surveys, regional surface geological mapping and hydrochemical and hydrogeological analysis. This paper describes the transport model itself and how, by means of a suite of supporting models, field data are translated into input parameters. Supporting models include: (i), a conceptual model of the proposed siting region; (ii) a set of hydrogeological models, the formulation of which is guided by the conceptual model and by hydrochemical and isotopic data. Sources of uncertainty in the conceptual model are described and a parameter sensitivity study is presented that illustrates the resultant uncertainty in radiological consequences. A particular source of uncertainty is the heterogeneous nature of the geosphere, giving rise to a complex and incomplete dataset from which ranges of input parameter values must be derived.

THE ROLE OF WATER-CONDUCTING FEATURES IN THE SWISS CONCEPT FOR THE DISPOSAL OF HIGH-LEVEL RADIOACTIVE WASTE

Crystalline basement rocks are considered as potential host formations for the disposal of radioactive waste in several countries. In northern Switzerland, six boreholes were drilled into the sediment-covered basement, and water-conducting features intersected by the boreholes were identified by hydraulic testing methods. The study of the corresponding core materials resulted in the distinction of three geological types of water-conducting features: faults, fractured zones and fractured aplite/pegmatite dykes. Conceptual models describing the spatial arrangement of channels (in which advection occurs) and of wallrock domains (where matrix diffusion and sorption occur) within the water-conducting features were derived. The migration of radionuclides released from a repository through water-conducting features was modelled, taking into account advection/dispersion, matrix diffusion, sorption and radioactive decay/ingrowth. Due to the diversity of the types of water-conducting features and of the small-scale geometric parameters, six model cases were considered that spanned the range of geometric parameter uncertainty. Calculated radiological doses are relatively insensitive to variations, within the ranges of uncertainty, of pathlength within the geosphere, longitudinal dispersion and the depth of the diffusion-accessible wallrock matrix. However, the small-scale spatial arrangement of channels within water-conducting features was identified as affecting the barrier function of the geosphere to radionuclide transport more strongly. For a given large-scale permeability, highest radionuclide fluxes from the geosphere were obtained for the model case that minimizes the spatial density of channels.