Comparison of alternative modelling approaches for groundwater flow in fractured rock

In performance assessment studies of radioactive waste disposal in crystalline rocks, one source of uncertainty is the appropriateness of conceptual models of the physical processes contributing to the potential transport of radionuclides. The Alternative Models Project (AMP) evaluates the uncertainty of models of groundwater flow, an uncertainty that arises from alternative conceptualisations of groundwater movement in fractured media. The AMP considers three modelling approaches for simulating flow and advective transport from the waste canisters to the biosphere: Stochastic Continuum, Discrete Fracture Network, and Channel Network. Each approach addresses spatial variability via Monte Carlo simulation, whose realisations are summarised by the statistics of three simplified measures of geosphere performance: travel time, transport resistance (a function of travel distance, flow-wetted surface per volume of rock, and Darcy velocity along a flowpath), and canister flux (Darcy velocity at repository depth). The AMP uses a common reference case defined by a specific model domain, boundary conditions, and layout of a hypothetical repository, with a consistent set of summary statistics to facilitate the comparison of the three approaches. The three modelling approaches predict similar median travel times and median canister fluxes, but dissimilar variability. The three modelling approaches also predict similar values for minimum travel time and maximum canister flux, and predict similar locations for particles exiting the geosphere. The results suggest that the problem specifications (i.e. boundary conditions and gross hydrogeology) constrain the flow modelling, limiting the impact of this conceptual uncertainty on performance assessment.