In-situ interference and tracer tests in granite rock with fractures at Beishan exploration tunnel, China

Fractures in the host rock surrounding a high-level radioactive waste (HLW) repository can act as primary conduits for groundwater flow and radionuclide migration, significantly impacting the repository's safety and long-term stability. Therefore, precisely characterizing the hydrological properties of these fractures is essential. This case study investigates fracture hydrology at the Beishan Exploration Tunnel, a key field platform for China's HLW repository in crystalline rock. A series of field investigations were conducted in six strategically positioned vertical boreholes intersecting faults. Methodologies included core logging, borehole acoustic televiewer logging, slug interference tests, natural gradient tracer tests, and dipole tracer tests. These integrated approaches were employed to assess critical hydrological parameters such as hydraulic conductivity, natural flow direction, inter-borehole hydraulic connectivity, and transport properties. The investigations identified two distinct sets of visible fractures. Slug interference tests provided hydraulic conductivity estimates for the boreholes, and inter-borehole hydraulic connectivity was evaluated using a proposed Hydraulic Connectivity Coefficient index, the centrally located borehole (A2-3) exhibiting the strongest connectivity. Natural gradient tracer tests established groundwater flow from borehole A2-5, through A2-3, to A2-1. Dipole tracer experiments, coupled with normalized mass flux data fitting, yielded key transport parameters including the number of flow channels, channel length, dispersivity, Darcy velocity, and tracer recovery rates. This comprehensive field characterization provides crucial site-specific data on fracture network properties and heterogeneity, demonstrating the efficacy of the integrated methodological approach. The results offer valuable insights for understanding potential radionuclide migration pathways and contribute to the safety assessment of the proposed HLW repository.