DIRECT AND INVERSE COUPLED THERMAL-HYDRO-BIO-GEOCHEMICAL MODELS FOR POROUS AND FRACTURED MEDIA
Creators: name: Yang, Changbing
name: Molinero, Jorge
name: Bonilla-Fernandez, Mercedes
name: Puigdomenech, Ignasi
name: Samper, Javier
name: Yang, Changbing
Publisher: name: XVI International Conference on Computational Methods in Water Resources
Year: 2006

description: 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.