Evaluation of two conceptual approaches for groundwater flow simulation for a rock domain at the block-scale for the Olkiluoto site, Finland

To evaluate conceptual approaches for simulating a rock domain, which is a rock unit outside a fracture zone, the block-scale groundwater flow system of the radioactive waste disposal site in Olkiluoto, Finland, was simulated using two approaches: the equivalent porous medium (EPM) approach and the hybrid approach. For the EPM approach, the rock domain was considered to be a homogeneous continuum with an equivalent hydraulic conductivity, while in the hybrid approach, it was conceptualized as a heterogeneous continuum from the stochastically generated background fracture network. In both approaches, several large fracture zones were included in the simulation models, which were calibrated by comparing the simulated hydraulic heads to the observed ones. The results showed that the simulated heads were in good agreement with the observed ones in both approaches, although the root mean square errors of the EPM approach were smaller than those of the hybrid approach. The estimated groundwater flow pathways in the hybrid approach generally had more complex geometries and longer travel times than those from the EPM approach. The simulations were evaluated by comparing the simulated flow rates at the boreholes in the site to the observed ones from the flow loggings, and the results show that the hybrid approach reproduced the observations more similarly than did the EPM approach. (C) 2015 Elsevier B.V. All rights reserved.