English Abstract 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.
