The most critical issue of the hydro-DFN-model is that only a subset of geologic fractures contribute to the groundwater flow. The authors deem that this is caused by the multiple episodes of sealing and reactivation (re-opening) of fractures. As the result of multiple episodes, groundwater flow-compartments are also formed. Flow-compartments are identified not only in the Cretaceous granodiorite in the Kamaishi mine in Japan but also in the Proterozoic granitoids of the Äspö Hard Rock Laboratory in Sweden. Understanding these processes demand the additional consideration in both site characterization and assessment. As to site characterization, identifying fracture filling minerals indicating conductive fracture such as euhedral calcite which precipitates from the present groundwater is important. Also, the pressure interference test which observes pressure responses during drilling of new holes is important to identifying flow-compartments. Sensitivity analysis and uncertainty analysis is highly recommended, since there is a practical limit for site characterization to characterize details of forementioned features for a large target rock volume. In the analysis, it is extremely important to assign reasonable constraint such as the number of conductive fracture/pathways, and the probabilistic distribution of flow rate and the maximum flow rate of the flow-paths identified by flow logging method.

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