Technischer Bericht NTB 90-14

Hydrogeologische Modellierung im Opalinus-Ton und in der Unteren Süsswasser-Molasse der Nordostschweiz

With the Sediment Study, Nagra is conducting research into deep disposal of high-level radioactive waste in sedimentary formations as an alternative to the crystalline option. In the first phase, a general assessment of all potentially suitable strata led to the identification of two most promising formations: the Jurassic Opalinus clay (OPA) and the Tertiary Lower Freshwater Molasse (USM). Subsequently, a multiregional site evaluation was conducted in order to identify tectonically and seismically quiet areas. The investigation concept adopted for this purpose as well as the evaluation results are documented in detail in the Sediment Study Report 88 (Nagra 1988). An important part of the assessment process is characterization of the expected regional groundwater flow. For this purpose, a numerical hydrogeologic model was developed for each selected region of first priority. The objectives of the modeling are to provide information about the probable regional distribution of hydraulic heads, the locations of recharge and discharge areas as well as the resulting flow directions and hydraulic gradients. Since both investigated regions are characterized by lack of hydrogeological data, the modeling shall also disclose the extent of uncertainties related to any assessment of the regional groundwater flow.

The model OPA 88 covers an area of 1'600 km2 and is composed of 1'600 finite elements. The model area includes the selected OPA regions of the Tabular Jura north of Lägern, Zürcher Weinland and Reiat. The adopted model parameters represent the current "best guess" and were subjected to variations in the sensitivity study in order to cover the associated uncertainties and assess their impact on simulation results.

The head distribution within the Opalinus clay is governed by the contrast in potentials between the regional aquifers of the Upper Malm (above) and Upper Muschelkalk (below). According to the orientation of the vertical hydraulic gradient, the groundwater percolating the host rock is drained either by the Malm or by the Muschelkalk aquifer and exfiltrates in the corresponding discharge zone. In the model area the following discharge locations of regional significance were identified:

  • the Rhine valley at Kaiserstuhl and Neuhausen (basin of the Rhine fall) for the aquifer of Upper Malm
  • the Rhine section at Koblenz for the Muschelkalk aquifer; the eastern part of the model area might however be drained by the remote Neckar basin near Stuttgart

The evaluation of hydrogeologically suitable repository areas is based on a relative comparison of the simulated travel times of groundwater flow from the host rock to the biosphere. Only flow paths with origin within the required depth range for a potential repository are considered. The simulations indicate that the best-suited region shall be situated as far in the Southeast part of the investigated area as possible (maximum depth allowance!), i.e. remote to the regional discharge zones along the Rhine river. However, this part of the model area manifests the largest uncertainties with respect to the head distribution in the Malm and Muschelkalk aquifers due to complete lack of reliable test data. Hence, the hydrological model does indeed provide information about the probable groundwater flow regime in the region considered, however, without a reduction of the existing uncertainties, particularly in the aforementioned head distributions, the model results cannot be considered as reliable. For the same reason (i.e. lack of data), the established model can be calibrated only by acquisition of additional field data.

The second numerical model USM 88 developed in the scope of the Sediment Study considers the regional groundwater flow in the Lower Freshwater Molasse (USM) east of the Limmat valley. It covers the Molasse basin between Lake Zurich and Lake Constance over an area of 2'600 km2 and consists of 1'700 elements. Conditions similar to the OPA model are found with respect to the availability of hydrogeological field data and to associated uncertainties. A noteworthy exception is the aquifer of the Upper Marine Molasse (OMM) where several potentiometric test data are available from deep groundwater and geothermal exploration wells. Since the OMM heads were not used for the definition of the boundary conditions except for the narrow outcrop area along the eastern margin of the Molasse basin, the observed heads within the model area are used for comparison of the results of the different model runs performed.

Also here, the groundwater flow through the host rock formation is governed by pressure conditions in the adjacent aquifers of regional importance. These are the marine sandstones of the OMM above and the limestones of the Upper Malm below the host rock. The dominating discharge area of the OMM aquifer is the low-lying broad alluvial valley at the confluence of the Thur and Rhine rivers. Other important areas are located in the valleys of Töss (at Rorbas) and Limmat as well as at the Lake Constance outside the model region. The discharge zones of the Malm aquifer have been described already in the OPA model.

The simulated OMM potentials manifest in the Molasse basin (between the Limmat valley and Lake Constance) a flat and smooth surface which is levelled to the peripheral discharge areas in the North and the West of the considered region. Due to the resulting drainage effect of the OMM the prevailing flow direction in the underlying host rock is pointed upwards. The largest vertical hydraulic gradients in the USM arise beneath the low-lying OMM and USM outcrops in the valleys. Since the confined OMM aquifer shows mostly artesian pressures in the lowlands (manifested by exploration wells Aqui, Tiefenbrunnen, Mainau, Konstanz etc.), the occurrence of hydraulic short-cuts towards the surface along permeable faults is possible here. Downwards-orientation of groundwater flow through the host rock can be observed only in the Southeast part of the model area in the hilly region of the Hörnli fan. Accordingly, the particle tracking results provide the longest travel times for groundwater flow paths originating in this area. The model simulations suggest hence that the most-promissive area for a potential repository site is located as far towards the Southeast of the investigated area as possible, i.e. remote to the erosion margins of the OMM and USM strata. Given the SE-oriented dip of the Molasse strata, this requires the repository to be sited at the maximum depth feasible.