2014:47 Technical Note, Assessment of groundwater salinity evolution at repository depth and especially the impact of dilute water infiltration

Groundwater compositions entering deposition holes in the repository will be progressively diluted over time because of expected changes of topographic and climatic conditions at Forsmark.  This is a long-term safety issue because very dilute water could destabilise compacted bentonite and erode buffer, leading to advective movement of sulphide-containing water to the canister surface.  SKB’s position in SR-Site is that the hydrogeological properties of the sparsely-fractured bedrock and the processes of water-rock reaction in shallow infiltration, hydrodynamic mixing  and exchange with pore waters in rock matrix will be such that only a very small number of deposition holes would potentially suffer advective dilute water conditions leading to buffer erosion.  This position is based on the results of complex modelling of groundwater flow and salt transport.  Conceptualisation and formulation of the models, including those describing the fracture network, inevitably require various assumptions and simplifications.  SKB has assessed the impact of these by sensitivity analyses and finds the forecasts of salinity evolution to be adequately robust.

Evolution of groundwater salinity at repository depth in the long term at Forsmark will be controlled by various bedrock properties and environmental water compositions.  Bedrock properties that will influence the penetration of dilute water into deposition holes include fracture patterns, transmissivity, connectivity, dispersivity and matrix diffusivity.  The degree of dilution of that water will be controlled by compositions of present groundwaters and pore waters in fractures and rock matrix and of infiltrating waters during future temperate and glacial climates and by the processes that modify and mix these components.