Modelling subrosion and groundwater flow in the vicinity of the Zuidwending diapir on the basis of geometrical and (palaeo)hydrological boundary conditions for the northeastern part of the Netherlands. (Final report of the SESAM project)

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The SESAM project was intended to perform a local model validation using site specific geological and (palaeo)hydrological boundary conditions in relation to the release of radionuclides from a waste repository in rock salt into the geosphere. The project was conducted by the RIVM, RGD, and IGG-TNO. Groundwater flow models are constructed for two selected areas: A regional model of the subsurface of the NE part of the Netherlands and a local model of the surroundings of the Zuidwending diapir. The models are constructed on the basis of geological and palaeo-environmental information, provided by the RGD. The steady-state regional groundwater flow for the present time and six Late Quaternary geological periods in the northeastern part of the Netherlands are simulated by IGG-TNO with the slightly adapted MODFLOW code. These studies provided the side flow boundary conditions for the local RIVM model, in which flow and transport in the vicinity of the selected diapir are examined with the METROPOL-3 model. Similar to current international standards with respect to safety assessment studies, the local model consists of i) a conceptual model, ii) a computer code, and iii) site-specific data sets. Emphasis has been put on calibration for present day conditions, detailed sensitivity analyses, simulation of flow and transport during six Late Quaternary geological periods, and a comparison of calculated subrosion rates with values reported in the literature. The simulations for the regional model showed that the potential gradients in the interglacial periods are considerably less than those in the periglacial periods. The largest groundwater velocities occurred during the Late Saalian. Application of the 3D local model resulted in an asymptotically decreasing subrosion rate of 0.12 mm/year after a simulation period of 50.000 years. The order of magnitude of the subrosion rates obtained with the METROPOL-3 code is the same as reported for other Dutch and German diapirs on the basis of geological methods. A sensitivity analysis indicated the side boundary fluxes and the permeabilities of the geological layers as te most sensitive parameters. As a consequence of the limited availability of data, complete validation of the local site-specific model is considered very difficult. For a meaningful calibration more filed data in the vicinity of the salt diapir are needed. Most essential are new data on the salt mass fraction distribution near the dome.