Capability for Hydrogeochemical Modelling within Discrete Fracture Networks

Abstract

A new method for simulating solute transport and geochemical interactions within fractured rock is presented. This will be an important capability for assessing the safety of radioactive waste disposal facilities that are located within fractured crystalline bedrock. Specifically, the discrete fracture network (DFN) module within the ConnectFlow groundwater flow and transport software has been updated to: (i) simulate the advection and diffusion of more than one solute species (with the flow and transport equations coupled by the evolving density and viscosity); (ii) model the diffusion of solutes into the rock matrix between fractures; and (iii) utilise the iPhreeqc library to model chemical reactions involving solutes, minerals on fracture/pore surfaces and rock minerals. The performance of ConnectFlow’s DFN module has also been significantly improved via parallelisation which allows more complex calculations to be attempted. These developments are significant because hydrogeochemistry within fractured rock is more accurately represented in an explicit DFN, rather than using more approximate equivalent continuous porous medium (ECPM) methods. Illustrative calculations have been completed for the disposal facility for spent nuclear fuel at Olkiluoto in Finland, and the former candidate site for spent fuel disposal, Laxemar, in Sweden. These calculations show that DFN simulations provide results that are qualitatively similar to results from ECPM calculations. However, because the ECPM is a less direct approach, notable differences exist when compared to the DFN approach.