DIRECT AND INVERSE COUPLED THERMAL-HYDRO-BIO-GEOCHEMICAL MODELS FOR POROUS AND FRACTURED MEDIA

It is well known that microbials play a major role controlling many redox-sensitive geochemical processes. In the context of radioactive waste disposal it has been recognized recently the need to evaluate the relevance of microbial processes in the performance of a HLW repository. A large amount of data on the structure, diversity and activity of native microbial populations has been collected in underground laboratories such as those in deep Swedish granites. Laboratory and field experiments such as the REX experiment have confirmed the relevance of microbial processes and have been useful to obtain relevant hydrochemical and microbial parameters and test conceptual biogeochemical models. Here we present direct and inverse models for coupled flow, reactive solute transport and biogeochemical processes in porous and fractured media. The inverse problem is formulated as the minimization of a generalized least-squares criterion by means of a Gauss-Newton- Levenberg-Marquardt method. Different types of data can be taken into account including hydraulic heads, aqueous and total concentrations, water fluxes and water contents as well as prior information on model parameters. The methodology can cope with the simultaneous estimation of flow, transport, microbial and geochemical parameters as well as initial and boundary waters, such as pH, pE, and abiotic and biotic concentrations. The mathematical formulation of inverse THBG models has been implemented in a finite element code, INVERSE-BIOCORE2D, which has been verified with synthetic examples and tested for the estimation of microbial parameters from the in situ REX experiment performed at a packed section of a borehole drilled from the access tunnel in Äspö Hard Rock laboratory in Sweden The hydro-biogeochemical model of the REX experiment considers microbially-mediated methane oxidation and organic matter respiration. Numerical results indicate that aerobic respiration of organic matter is much more efficient in consuming dissolved oxygen than pyrite dissolution. Initial concentration of heterotrophic bacteria and their growth rate constant are the two most sensitive parameters. Calibrated THBG numerical models have been later used for testing quantitatively several scenarios for a HLW repository in Sweden including: 1) Aerobic respiration associated to the oxygen supplied during the pre-closure stage; 2) Hydrochemical perturbations caused by the construction of the repository during its operational stage; and (3) Scenario of a glaciation in which melting waters rich in oxygen and no organic matter might reach the repository. Results of these models should be relevant for the performance assessment of a HLW repository in granites for the long-evolution of groundwater chemistry and redox conditions during pre- and post- repository closure.