In deep underground areas where fracture connectivity is assumed to be low (*1), the entire rock matrix (including all voids) is treated as the primary water pathway. In contrast, where fracture connectivity is assumed to be high (*2), fractures are treated as the water pathway, and effective porosity is calculated based on the fracture zones (*3).

(a) The modeling domain and geological cross-section of the observation points. (b) Results of numerical analysis of groundwater travel time: (1) When matrix porosity is assigned to the Koetoi and shallow Wakkanai formations, and (2) when fracture porosity is assigned to the same formations.

In the site selection process for high-level radioactive waste disposal, regional-scale groundwater flow modeling—spanning several to tens of kilometers from recharge^*4 to discharge zones—is employed to evaluate groundwater travel times and flow paths. Among the parameters influencing travel time estimation, effective porosity, defined as the proportion of voids through which groundwater can flow within the bedrock, is particularly sensitive to changes.

In sedimentary formations, fracture permeability derived from hydraulic testing during borehole investigations is often used to represent the permeability of bedrock. However, the methodology for assigning effective porosity—whether based on the entire rock matrix or limited to fracture zones—remains ambiguous.

This study focuses on the Koetoi and shallow Wakkanai formations in Horonobe, Hokkaido, which are sedimentary rocks characterized by low-permeability rock matrices and developed fracture networks. Using effective porosity estimated from fracture widths (Fig. 1), groundwater travel times were calculated by numerical analysis and compared with groundwater age data obtained from borehole investigations. Given that the groundwater flow in the deep subsurface of this region may have occurred under conditions different from the present (Fig. 2a)—such as surface topography and recharge^*4 rates—the analysis focused on the shallow zone enclosed by red dashed lines () in Fig. 2b. The results showed that using fracture-width-based porosity estimates yielded relatively short travel times that were consistent with observed data from borehole investigations.

These findings suggest that, in sedimentary rocks where water pathways are primarily formed by fractures within a low-permeability matrix, estimating effective porosity based on fracture width is a valid and reliable approach for groundwater flow modeling.

 

^*4 Groundwater recharge refers to the process by which surface water, such as precipitation, infiltrates into the subsurface aquifers.

This research is part of the outcomes of the project commissioned by METI (FY2022, Grant Number JPJ007597), R&D supporting program titled "Research and Development on Groundwater Flow Evaluation Technology in Bedrock."

Miyakawa, K. et al., Effective Porosity in Groundwater Flow Analysis for Fractured Sedimentary Rocks; Case Studies of the Koetoi and Wakkanai Formations in Horonobe, Hokkaido, Japan, Journal of Nuclear Fuel Cycle and Environment, vol.31, issue 2, 2024, p.82–95.

Paper URL: https://doi.org/10.3327/jnuce.31.2_82 (in Japanese).

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