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Recent publications

Workreport 2020-1



Modelling Solute Transport and Water-Rock Interactions in Discrete Fracture Networks


Applegate, D., Appleyard, P., Joyce, S.



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The radioactive waste disposal facilities at Olkiluoto in Finland and at Forsmark in Sweden, will be located in higher strength crystalline rock, where the groundwater is largely constrained to flow through sparse fracture networks. The chemical composition of the groundwater will play a key role in determining the safety of these geological disposal facilities, particularly the chemical stability of the buffer material and the resilience of the waste canisters. Chemical factors that are relevant for safety assessments include salinity, redox potential, pH and the concentration of a number of key chemical components.

In the past, safety assessments have followed one of two approaches to assess the hydrogeochemistry within fractured rock: (i) using an equivalent continuous porous media representation of the fracture network to model the transport and chemical interactions; or (ii) calculating steady state pressure solutions utilising explicit discrete fracture network models, then inferring the transport properties of the fracture network utilising particle tracking information. This report presents a new methodology, where the hydrogeochemical evolution is modelled using an explicit fractured rock representation. This has been accomplished by extending the discrete fracture network facility for the ConnectFlow groundwater flow and transport software to: (i) model solute diffusion into the pore space of the surrounding rock (i.e. matrix diffusion); (ii) solve the advection-diffusion equation for multiple solute species rather than just a single species (which is fully coupled to the pressure solution via the buoyancy term in Darcy’s equation); and (iii) calculate chemical reactions of the groundwater solutes, possibly involving fracture/pore surface minerals or rock minerals, utilising an interface to the iPhreeqc library. Software parallelisation has significantly improved the performance of the code and extended the size of problems that can be solved.

The explicit representation of reactive transport within discrete fracture network models is a significant advance that allows groundwater flow, transport and hydrogeochemical reactions to be properly represented in a fractured bedrock. Preliminary hydrogeochemical calculations are presented for the ONKALO disposal facility at Olkiluoto, and the proposed Swedish repository for long-lived waste, SFL. These calculations show that DFN similations provide qualitatively similar results to those obtained from an ECPM representation. The ECPM is a more indirect approach, however, and differences do exist between the results from the DFN and the ECPM representations. These differences typically reduce as the resolution of the ECPM grid is increased.


Crystalline rock, discrete fracture network, geological disposal, safety assessment, matrix diffusion, reactive transport


WR 2020-01_web (pdf) (4.3 MB)


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