Numerical Study on the Macroscopic Mass Transport Properties of Fractured Shale with Matrix Diffusion: Forward and Inverse Modeling based on Laboratory Tracer Test and In-situ Well Test Results at Horonobe URL Site, Hokkaido, Japan.

Understanding the long term evolution of groundwater is essential for siting and performance assessment of HLW repositories. Japan Atomic Energy Agency, JAEA, is carrying out extensive geoscience study for HLW disposal in the sedimentary rock formation at Horonobe URL site, Hokkaido Japan. In the URL site, fractured siliceous mudstone formations, the Koetoi formation and the Wakkanai formation, are overlain by the Quaternary layer. From geochemical analyses, it was found that in the shallower groundwater region, where interglacial period rainwater dominates, the Cl- concentration of squeezed core water (matrix water) was much higher compared to that of pumped groundwater (fracture water). Whereas in the deeper zone containing glacial period rainwater, both squeezed and pumped water showed the same salinity. In this study, the authors conducted a numerical study on the evaluation of macroscopic mass transfer properties, required for the planned regional scale mass transport simulation at the Horonobe site, of fractured rock considering matrix diffusion. Modeling study consists of forward and inverse analyses. First, we carried out advection/dispersion mass transport analyses considering matrix diffusion using a regularly spaced fracture network model with various fracture densities. The transport parameters were determined by laboratory tracer tests and in-situ hydraulic tests. Then, an inverse analysis was carried out using two lumped parameter models, i.e., a homogeneous porous model and a single fracture added to a porous model (referred to as "a single fracture model"). The inverse analysis was carried out in two stages; identification of advection/dispersion properties using breakthrough curves obtained by a no matrix diffusion model followed by identification of the diffusion parameters. As a result, the breakthrough curve obtained from a fracture network model cannot be reproduced by a homogeneous porous model, whereas both breakthrough curve and mass balance can be sufficiently reproduced using a single fracture model. A correlation between advection/dispersion parameters and the fracture density was also confirmed. Further study is needed, however, it was shown that the macroscopic transport properties may exist and can be evaluated by the proposed forward/inverse modeling approach.