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