Get the current state right first – lessons learnt from a groundwater modelling perspective

Groundwater models considering density-dependent flow are one of the instruments used to assess post-closure safety of deep geological repositories (DGR). Their task is to assess potential flow paths of radionuclides or other pollutants migrating from the DGR to the near surface environment through long-term prognoses. Often a one million years period is assumed for these long-term prognoses.

Scientific discussions on this topic focus very often on questions whether to include further processes (thermal, mechanical) or how the climate will develop in the next one million years. Somehow, a bit out of focus are the past and the current state where we in the best case have measured values and know much more than about the future in a million years. An important lesson learnt is to get the current state of the hydrological and hydrogeological system right as the main precondition for qualified long-term prognoses. Using examples from the modelling practice, the following aspects will be highlighted.

The analysis should start with a thorough understanding of the near surface groundwater flow system and its main driving forces. On the one hand, a transient groundwater recharge boundary condition is required. It is indispensable for a transient calibration which should be state-of-the-art nowadays. On the other hand, the partially saturated zone and the surface water groundwater interaction need to be described by the model. As the leakage iterations require extra computing time and the focus is shifted too much to the deep groundwater system this part is often omitted. The near surface groundwater flow system influences the deep groundwater flow system and vice versa. Usually, the near surface processes are described through field observation much better than the deep groundwater. Building up a transient 2D model as a start enables a good system understanding from the beginning. After that, the model is upgraded to a fully 3D model including all relevant hydrogeologic units and eventually discrete fractures. But also the 3D model has to include a transient groundwater recharge, the unsaturated zone and the leakage interaction with surface waters. The steady-state calibration using a mean groundwater recharge is the starting point but the model can only be used for prognoses after a good transient calibration result.

Last but not least, the software code used for this kind of analyses should be regularly verified using standard verification examples for flow and density dependent flow and transport.