Relationship among the fault transmissivity, flow dimension, and effective hydraulic conductivity in siliceous mudstone of the Wakkanai Formation around the Horonobe Underground Research Laboratory in Japan

Abstract

Effective hydraulic conductivity is typically used for evaluating groundwater flow in faulted/fractured media. Estimating effective hydraulic conductivity relies on modelling the transmissivity and hydraulic connectivity of faults/fractures from a large dataset. As an alternative for evaluating hydraulic connectivity, flow dimension is an easily available and quantitative indicator of fault/fracture hydraulic connectivity. However, the flow dimension has not been used quantitatively as much to evaluate the effective hydraulic conductivity. This study demonstrates how flow dimensions in poorly self-sealed faults directly relate to the effective hydraulic conductivities. The effective hydraulic conductivities in massive siliceous mudstone of the Wakkanai Formation around the Horonobe Underground Research Laboratory in Japan were analysed by simulation with long-term (over 10 years) data of hydraulic pressure and inflow during its construction and operation. The estimated effective hydraulic conductivities varied from ≈10 −8 to ≈10 −11 m s -1 with an increasing depth from ≈250 to ≈600 m. The fault local transmissivities and flow dimensions from packer tests varied from ≈10 −6 to ≈10 −10 m 2 s -1 and from ≈2.0 or more to ≈1.0 with increasing depth, respectively. When applying Landau–Lifshitz–Matheron's formula to calculate the fault effective transmissivities, these variations correlate well, and the estimated effective hydraulic conductivities are sound. Thematic collection: This article is part of the Sustainable geological disposal and containment of radioactive waste collection available at: https://www.lyellcollection.org/topic/collections/radioactive