Modelling of the Tasks 6A, 6B and 6B2 using Posiva streamtube approach

This report summarises the modelling work of the Tasks 6A, 6B and 6B2. All tasks are modelled using analytical one-dimensional Posiva streamtube approach. This approach assumes that transport takes place through a channel or channels. Processes taken into account in the modelling are advection through the channel(s), sorption on the fracture walls, matrix diffusion to stagnant pools or fault gouge or rock matrix and sorption in the stagnant pool or pore spaces of fault gouge or rock matrix.

Channel geometry defines the flow field and in that way influences the matrix diffusion properties. Channel geometry has been one of the calibration parameters used in Task 6A to fit the model with the measured in-situ tracer test breakthrough curves. In Tasks 6A and 6B a single channel of aperture 2 mm, width 25 mm and length 5.03 m is used. Flow rate and sorption, diffusivity and porosity properties are taken from the data given in the task definition.

Task 6A modelling uses three different immobile pore spaces: stagnant pools, fault gouge and rock matrix. Simulations of the tracer penetration depth in different pore spaces indicate that stagnant pools and fault gouge get saturated in Task 6B conditions. Therefore, Task 6B modelling uses only rock matrix as immobile pore space.

In Task 6B2 transport over larger area of the feature A in considered. Modelling is again based on the streamtube approach. In this case several parallel streamtubes are used. All streamtubes have similar geometry but flow rates vary. The flow rate distribution applied in the Task 6B2 is based on the flow rates measured in five different boreholes using dilution method.

Comparisons of the Task 6B and 6B2 show that if the transport takes place over larger area the breakthrough curves are more spread. Naturally, this comes from the larger spread of the different flow rates (or channels) the tracer encounter in larger region.