RD&D-Programme 2004 - SKB

In a continuation of the super-regional modelling of Northern Uppland, a more detailed study
has been done of groundwater flow near the ground surface, the so-called geosphere-biosphere
interface /19-18/. In this study, the Quaternary deposits have been modelled at high resolution,
at the same time as seasonal variations in groundwater infiltration have been included. The
results show that even though groundwater discharge areas exist over large parts of the model’s
upper surface, discharge of water particles from repository depth takes place in well-defined low
points. Such low points are typically lakes, below the sea, and other low points in the terrain.
This picture of discharge from repository depth is only marginally affected by not explicitly
including the Quaternary deposits. Further, it is shown that even though seasonal variations in
groundwater infiltration are of great importance for water flow in the Quaternary deposits, the
effect on flow patterns for particles released at repository depth is negligible. Finally, it is also
shown that the flow paths in the Quaternary deposits are relatively short in length (about ten
times longer than the thickness of the Quaternary deposits). The longest flow paths are often
beneath lake bottoms; the denser the bottom sediment, the longer these paths tend to be. This
study indicates that the modelling strategy that is normally used to describe the deep groundwater
is adequate for describing discharge patterns for radionuclides from repository depth with
sufficient accuracy. Coupled biosphere-geosphere models are thus not needed for this purpose.
Programme
During the coming three-year period, the two calculation tools for groundwater flow will be
used and tested within the site modelling projects and the safety assessment. Furthermore,
targeted activities are planned to shed light on individual issues. Such an issue is understanding
how the composition, in particular the salinity, of the groundwater has evolved and will evolve
in the future. All new hydrogeological and geochemical data that have become available since
the last official model version for Äspö are being compiled in a project connected to the Äspö
HRL, Äspö Models 2005 /19-19/. These data will be used together with DarcyTools to try to
provide a coherent picture of how the water chemistry (salt distribution) has evolved and will
evolve in a 10,000-year perspective. A hypothesis is that diffusion of salt into the rock matrix is
an important process to bear in mind for understanding the historic salt evolution. The results of
this study are expected to be directly applicable to the site modelling projects.
Additional resources will also be devoted to an improved understanding and development of
modelling tools for near-surface hydrology. Two parallel tracks will be followed in this phase:
a simplified version and a coupled hypothesis. In the simplified version, it is assumed that the
models for the deep groundwater provide sufficient information and accuracy for predicting the
release points of radionuclides in the biosphere, see above discussion. In this case, all that is
needed is a model for the surface water hydrology that provides relevant information regarding
what dilution can be expected to occur with surface water. However, these processes must be
correctly conceptualized and implemented in the models. This simplified approach should be
compared with a coupled strategy where both the deep and near-surface groundwater flow, as
well as the surface water hydrology, is described in the same model. An open question is how
detailed the biosphere description in these models should be, i.e. can the biosphere processes be
described in a separate model, or should they also be incorporated in the coupled hydrological
model for groundwater and surface water? These types of studies are expected to be conducted
as an integral part of the site modelling work.
Some model development is expected to take place in the coming years, for example development
of boundary condition options for simulation of an open repository so that the effect of a
groundwater lowering in the near-surface soil layers can be studied. In general, the modelling
tools should be tested more rigorously for applications with an open repository, since studies of
this type have not been conducted previously to any appreciable extent.
The development of Connectflow is being pursued effectively via multilateral cooperation in the
iConnect Club. Our hope is that this activity will continue during the coming three-year period.
A doctoral project will be initiated where more fundamental issues relating to processes such as
density-driven flow and two-phase flow in discrete fracture networks can be investigated.
248 RD&D-Programme 2004