Inverse hydraulic and transport model of groundwater recovery experiment using mixed-dimensional concept

Understanding evolution of groundwater hydraulic and chemical conditions is important for safety assessment of radioactive waste disposal. Perturbations to groundwater can occur due to the construction and operation of the underground facility. The initial perturbations and their recovery after the tunnel closure were observed in the Groundwater REcovery Experiment in Tunnel (GREET) in Mizunami, Japan, at 500-m depth in granite. In a 100m-long tunnel, 50 m was isolated by building a plug. Aside from other measurement and exploration data, groundwater pressures and chemical composition were monitored in boreholes with 24 packer sections. This work used numerical modelling to better understand and be able to predict the observed water flow and solute transport phenomena. The model covered 100-m scale around the tunnel and was divided into a continuum farfield domain and a near-field domain with deterministic discrete fractures and matrix blocks. A mixed-hybrid finite element solution was used with independent degrees of freedom for 3D and 2D elements. Modelling started with a blind prediction followed by a calibration (inverse model) for drainage and flooding experiment phases. The inverse hydraulic model fitted the more/less communicating sections and estimated transmissivity and conductivity parameters that are consistent for the two phases. The model sensitivity on transport parameters was insufficient for the inverse model, which can use only simplified measured evolution to avoid noisy data.