Microbial Diversity at Iron-Clay Interfaces after 10 Years of Interaction Inside a Deep Argillite Geological Formation (Tournemire,France)

In the context of a geological disposal of radioactive waste in clayey formations, the consequences of microbial activity are of concern regarding the corrosion of metallic components. The purpose of the present work was to characterize the microbial diversity that may have impacted corrosion processes at the interface between re-compacted argillite and steel coupons after 10 years of interaction inside the Toarcian argillite layer in Tournemire (France) under in situ conditions. Several types of steel coupons were introduced in 1999 in two boreholes (so-called CR6 and CR8) drilled in the geological formation and filled with re-compacted argillite. CR6 borehole was drilled horizontally from a century-old railway tunnel and coupons were placed in the undisturbed argillite. CR8 borehole was drilled vertically under the tunnel, in conditions influenced by the draining of the Cernon fault water. CR6 and CR8 boreholes were overcored 10 years later and steel coupons as well as re-compacted argillite samples were analyzed separately. The characterization of their microbial diversity was carried out using culture media and molecular methods using 16S rRNA genes cloning. Data resulting from both approaches were complementary. Isolates and clone sequences were affiliated to only 3 bacterial phyla: Firmicutes, Actinobacteria and Proteobacteria. The biodiversities differed depending on the steel type and the borehole considered, indicating the influence of both iron-clay interactions and in situ environmental conditions. This analysis has highlighted the presence of sulphate-reducing bacteria, iron-reducing bacteria and isolates capable to develop at high temperatures. These microorganisms can grow at the interfaces between materials in a very short period of time compared with planned durations of disposal. Thus, these results should be considered to assess the consequences of microbial activities on the evolution of the metallic components like overpacks.