4 Description of the Disposal System for SF / HLW / ILW in Opalinus Clay
4.2 The geological environment of the repository
4.2.5 Water and solute movement in the host formation Hydraulic regime in Opalinus Clay
The potential for vertical movement of water across the sedimentary formations in the Zürcher Weinland is controlled by the hydraulic properties of the formations and the hydraulic driving forces. Fig. 4.2-8 indicates overpressures of 50 – 300 m (best estimate reference value 100 m based on hydraulic test interpretation) above hydrostatic head in the Opalinus Clay. Similar overpressures have been measured in the argillaceous horizons in the Lias and possibly also in the low permeability gypsum horizon in the Keuper below (see Figs. 4.2-6 and 4.2-8). The Wedelsandstein Formation exhibits significant underpressure (60 m below hydrostatic), showing it to be hydraulically separated from the overpressured formations above and below, whereas the Stubensandstein Formation is overpressured (60 m above hydrostatic). The hydraulic head values are close to hydrostatic within or near to the regional aquifers. There are, however, a limited number of hydraulic head measurements in the region, and this needs to be considered when discussing the flow regime.
Today, and until the overpressures are dissipated, the hydraulic gradient at the mid-plane of the Opalinus Clay is oriented vertically upwards and downwards in the lower parts. It is assumed that these overpressures are inherited from an earlier period of rapid burial or, alternatively, ongoing lateral stress, for which drainage and compaction of the Opalinus Clay has not yet resulted in a state of pressure equilibrium. The long-term effects of clay compaction on the performance of the geological barrier are discussed in Section 5.2.2.
Water conducting
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Fig. 4.2-10: Potential groundwater transport paths based on siting information and hydro-dynamic modelling
a) lateral transport occurs in minor aquifers in confining units, b) discharge occurs through the regional aquifers after vertical transport through upper and lower confining units, c) after lateral transport through minor aquifers in confining units for at least 1 km (200 m in the pessimistic case), discharge occurs through regional aquifers due to a large fault connecting the confining unit and regional aquifers.
There are various ways of characterising the vertical hydraulic pressure gradient across the host rock formation, which can be treated as variants in subsequent sensitivity analysis calculations.
At the largest scale, the hydraulic gradient within the Opalinus Clay will be controlled by the head difference between the two regional aquifers. This will also be the case in future, once the overpressures are naturally dissipated. Currently, the observed hydraulic head difference between the regional aquifers is relatively small, producing a hydraulic gradient of 0.05.
At a smaller scale, the gradient across the host formation is controlled by the head difference between the Stubensandstein Formation (60 m above hydrostatic) and the Wedelsandstein Formation (60 m below hydrostatic), resulting in a gradient of about unity. Yet on a more local scale, a gradient of 5 might be considered between the even more highly overpressured host rock formation and the Wedelsandstein Formation. Active hydraulic connections between the regional aquifers or the discontinuous water-conducting formations may exist only through sub-vertical water-conducting faults, and no such features have been observed in the potential repository area (Fig. 4.2.5).
* 60 m if Keuper sandstones are effective (indication from Benken borehole) 30 m if Arietenkalk is effective (no indications in Zürcher Weinland)
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Scenarios for hydraulic gradients that could drive vertical advection are indicated in Tab. 4.2-3.
Tab. 4.2-3: Hydraulic gradients (in m m-1) between different formations in the Zürcher Weinland sedimentary rock sequence
The Opalinus Clay has an extremely low hydraulic conductivity: From the field tests in Benken and from core samples typical hydraulic conductivities in the range 10-13 to 10-14 m s-1 have been determined, suggesting a best estimate of ~10-13 ms-1 parallel to the bedding and ~2×10-14 m s-1 normal to the bedding. These values are also consistent with tests and long-term observations from Mont Terri and with isotopic profile data (see below). It is worth mentioning that hydraulic tests are carried out at elevated hydraulic gradients (typically > 50), which are not observed in the natural system. Furthermore, these values represent rock properties on a relatively small scale and, therefore, other observations are used to test the applicability of these conductivities at larger scales ("upscaling"). For this purpose, the observed overpressures were modelled with specific basin models investigating different possible mechanisms for generating overpressures (effect of consolidation from rapid burial, effect of lateral thrust). The results of these models clearly indicate that if the overpressures represent the large-scale porewater pressure in the host rock formation, then the hydraulic conductivity of the Opalinus Clay has either to be very low (10-15 m s-1 or lower) or a non-Darcian flow regime exists where a threshold gradient has to be exceeded before flow starts or – most likely a combination of both a very low hydraulic conductivity and a threshold gradient. If one or both of these were not the case, then the observed overpressures could not be sustained over geological time scales. The existence of such threshold gradients is consistent with all existing observations but cannot be quantified with confidence. Conductivities of 10-15 m s-1 or lower cannot easily be justified because measurements indicate higher values. A value of 10-13 m s-1 is considered an upper limit. In any case, whether or not the overpressures are real, the water is quasi-stagnant.
A hydrodynamic model, discussed in detail in Nagra (2002a), was developed to help in the interpretation of the local hydrodynamic situation. The apparent inconsistency between measured conductivities and observed overpressures and our current inability to quantify threshold gradients with confidence necessitated the use of pessimistic parameters in the model.
In all calculational cases, the measured hydraulic conductivities of 10-13 and 2 × 10-14 m s-1 were used and threshold gradients were neglected, except for case RF2 where conductivities of 10-15 and 2 × 10-16 m s-1 were used (Nagra 2002a). To investigate the effects of existing uncertainties regarding boundary conditions and properties of some formations, different cases were modelled applying Darcy's law. The results show different vertical gradients through the Opalinus Clay, but in all cases, only a small vertical flux upward is observed through the repository horizon. The results for the spectrum of cases analysed with the hydrodynamic model are summarised in Tab. 4.2-4 and are briefly discussed below.
Muschelkalk to Malm aquifers
(ignoring high overpressure in Opalinus Clay) 0.05
Stubensandstein Formation to Wedelsandstein Formation
(ignoring high overpressure in Opalinus Clay) 1
Centre of Opalinus Clay (repository horizon) to Wedelsandstein Formation
(accounting for high overpressure in Opalinus Clay) 5
Formation Hydraulic gradient
[m m ]-1
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Tab. 4.2-4: Key results from the hydrodynamic model for the different cases analysed (see also Nagra 2002a)
The smallest flow through the Opalinus Clay is observed for the case where a very low conductivity is allocated to the Opalinus Clay (case RF2). For the other cases (RF0, RF1, RF3, RF4, RF5, RF8), the flow rates are similar, except for those in which both the Neuhausen fault and the Wildensbuch flexure are given higher transmissivities (case RF6 - transmissivity of 10-7 m2 s-1, case RF7 - transmissivity of 10-5 m2 s-1), which leads to a slight decrease in the gradient across the Opalinus Clay. In Benken, the observed heads cannot be reconciled with these latter two cases, thus the transmissivity of these features must be smaller than 10-7 m2 s-1. Some further perspective on the validity of the measured hydraulic conductivity values for Opalinus Clay can be obtained by comparing the data with values obtained for other claystones as well as for shales and unconsolidated clays (Fig. 4.2-11). The data for Opalinus Clay are fully consistent with the trend of decreasing permeability with decreasing porosity.
Solute transport through the Opalinus Clay
With conductivities of 2 × 10-14 m s-1 (vertical) and 10-13 m s-1 (horizontal) or lower and the possible existence of a threshold gradient, the porewater will be effectively stagnant. For such stagnant porewaters, transport is dominated by diffusion.
Case Hydraulic conductivity Opalinus Clay
[m s-1]
Specific [m sflux-1]
Key characteristics
KH KV
RF0 10-13 210-14 10-14 Regime determined by Malm and Sandsteinkeuper RF1 10-13 210-14 10-14 Regime determined by Sandsteinkeuper, and
enhanced permeability in Wedelsandstein RF2 10-15 210-16 10-16 Regime determined by Wedelsandstein and
Sandsteinkeuper, low K in Opalinus Clay RF3 10-13 210-14 10-14 Regime determined by Wedelsandstein and
Sandsteinkeuper (assumption: Sandsteinkeuper exfiltrates towards Neckar)
RF4 10-13 210-14 10-14 Overpressures in Opalinus Clay
RF5 10-13 210-14 910-15 Regime determined by Wedelsandstein and Sandsteinkeuper
RF6 10-13 210-14 410-15 Regime determined by Wedelsandstein and Sandsteinkeuper; Neuhausen fault: K = 10-8m s-1; Wildensbuch flexure: K = 10-8m s-1
RF7 10-13 210-14 410-15 Regime determined by Wedelsandstein and Sandsteinkeuper; Neuhausen fault: K = 10-6m s-1; Wildensbuch flexure: K = 10-6m s-1
RF8 10-13 210-14 210-14 Heads calibrated to measured values in Benken borehole (inverse modelling)
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Fig. 4.2-11: Compilation of porosity and permeability data from argillaceous rocks of different maturity after Neuzil (1994) and comparison with results from the Benken bore-hole and the Mont Terri investigation tunnel
Fig. 4.2-12: Diffusion coefficient and porosity for tritium (perpendicular to bedding) in differ-ent argillaceous rocks (Nagra 2002a)
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Fig. 4.2-13: Texture and structure of the Opalinus Clay, illustrating characteristics on various scales that give rise to anisotropy in transport properties (see text and Nagra 2002a) The diffusion properties of Opalinus Clay have been investigated in both laboratory and field studies. The latter includes migration experiments and analysis of the large scale distribution of solutes and isotopes carried out in the Mont Terri Rock Laboratory. The obtained diffusion data show general agreement at different temporal and spatial scales (Nagra 2002a). The laboratory diffusion constants for Benken samples are somewhat lower than those obtained for Mont Terri samples (Van Loon et al. 2002 and 2003). The effective diffusion coefficients of tritium for Benken samples obtained from laboratory measurements are about 6 × 10-12 m2 s-1 perpendicular to the bedding plane. For anions, effective diffusion constants are 10 times lower. The diffusion constants parallel to bedding are about five times higher, consistent with the strong anisotropy of the medium. The diffusion accessible porosity for tritium was found to be in the range of 0.12 - 0.15, whereas anions show significantly lower values of about 0.06. Very few diffusion data exist for cations. Preliminary data (Van Loon et al. 2003) suggest that effective diffusivities for Na+ are somewhat higher (a factor of 2) compared to tritium. Comparison of diffusion data for Opalinus Clay with those from other clay formations reveals a correlation between diffusion
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constants and porosity. Thus claystones with a similar degree of consolidation, such as the Callovo-Oxfordian of Meuse/Haute-Marne and the Toarcian of Tournemire, show similar diffusion coefficients, as shown in Fig. 4.2-12.
The observed low conductivities and the anisotropies in conductivity and diffusivity are due to the structure of Opalinus Clay, which is illustrated in Fig. 4.2-13. On a medium scale (~ 1 mm) the clay particles with their plate-like geometry (length 10 – 1000 times their thickness), which on the average make up about 50 % of the minerals, are horizontally bedded. This bedding is responsible for the observed anisotropies in conductivity and diffusivity. On a nm-scale, the specific properties of the clay minerals and their interaction with the porewater are most important. Approximately 75 % of the pores have apertures in the range of 1 – 25 nm. This very fine pore structure is the reason for the very low hydraulic conductivities despite the significant water content of the Opalinus Clay. Because of the interaction of the porewater with the sur-faces of the clay minerals, only a fraction (approx. 50 %) is free water, the remainder being bound.
On a larger scale (10 m), fracture zones (faults) must be considered. Large faults are not present within the potential repository area (see Section 4.2.2), but smaller fracture zones cannot be ruled out. However, none of the discontinuities observed in Nagra’s deep boreholes at Benken and Schafisheim revealed enhanced permeabilities in comparison to the rock matrix. Faults, shear-zones and joints have been observed in Opalinus Clay at different locations and depths in tunnels intersecting the formation. However, significant transmissivities (~ 10-10 m2 s-1) are not observed at locations with overburdens larger than 200 m, a fact that can be explained by the efficient self-sealing capacity of Opalinus Clay (Gautschi 2001). A more detailed description of the properties of Opalinus Clay for the different scales can be found in Nagra (2002a).
Evidence for the high isolation capacity of the Opalinus Clay and its strong barrier function within the hydrogeological system of the sedimentary sequence comes from detailed analyses of porewater chemistry, and models of how this has evolved with time. Fig. 4.2-14 shows measured concentration profiles across the Opalinus Clay and adjacent rock strata of the two naturally occurring stable isotopes, 18O and 2H, from core samples from the Benken borehole.
The isotopes were assumed to be originally distributed approximately uniformly across the Opalinus Clay ~ 1 million years ago, before the current regional groundwater flow system was established. 18O and 2H have subsequently migrated outwards into the over- and underlying aquifers, modifying the uniform concentration profiles. The reason for the movement of these isotopes is changing water composition in the more permeable formations above and below, caused by flushing of the aquifers with younger waters. This caused a concentration gradient away from the centre of the claystone sequence, leading to diffusive transport of the isotopes outwards into the aquifers. The measured profiles are compared in Fig. 4.2-14 to model profiles for which it is assumed that diffusion has occurred for 0.25, 0.5 or 1 Ma. Calculations illustrate that significant deviations from the measured isotope profiles occur for flows in excess of 10-12 m s-1 (Gimmi & Waber 2003).
Fig. 4.2-14 provides compelling evidence for the dominant role of diffusion in controlling porewater compositions in the host clay formations and, by analogy, in controlling the move-ment of any radionuclides released into those porewaters from a repository.
Appendix 2 (Tabs. A2.8 and A2.9) summarises the key hydraulic parameters relevant to water and solute movement in Opalinus Clay in its present state. The potential evolution of these hydraulic properties over time, including the effects of repository-induced disturbances, is discussed in Section 5.5.3.
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Fig. 4.2-14: Isotope concentration profiles in porewater across the Opalinus Clay (OPA) and adjacent rock strata due to diffusion that occurred for 0.25, 0.5 and 1 Ma.
Results are based on measured data (Nagra 2001, 2002a) obtained under various conditions (data points) and modelling (Nagra 2002a, Gimmi & Waber 2003) assuming diffusion only, presented as δ values relative to an international standard water (V-SMOW).
-65 -60 -55 -50 -45 -40 -35
400 440 480 520 560 600 640 680 720
0.25 Ma 0.5 Ma 1 Ma
Malm
Lias Keuper
10 -9 -8 -7 -6 -5 -4
18O [ ‰]
2H [ ‰ ]
Lower Dogger (OPA+ MB) Upper + Middle Dogger
400 440 480 520 560 600 640 680 720
-Depthb.g.[m]Depthb.g.[m]
Malm
Lias Keuper Lower Dogger (OPA+ MB) Upper + Middle Dogger 0.25 Ma.
0.5 Ma 1 Ma
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