Safe disposal of radioactive waste is a critical challenge to ensure the safety for future generations of humans and the biosphere in general. Deep geological disposal has been considered in many countries as a potential safe long-term solution. A combination of engineered and natural barriers between the waste and the environment should prevent the radionuclides from reaching the surface in such concentrations that they could present an unacceptable risk for man and the environment. In Belgium, Boom Clay has suitable physico-chemical characteristics to serve as natural barrier for the disposal of radioactive waste, hence Boom Clay is studied in detail. One important aspect herein is the study of the geochemical environment within the Boom Clay, in particular the pore water composition and its pH and Eh, as this determines the speciation of radionuclides and hence their (im)mobility behaviour. Microbial activity may affect this geochemical enviroment, as microbial reactions may change the pore water composition and hence may affect the mobility of radionuclides. Therefore, it is necessary to study the effect of microbial activity on the pore water composition of the Boom Clay in different conditions representative for geological disposal: natural conditions, influence of oxidation, high pH, increased temperature or increased salinity.
Sampling of pore water is done in situ from various piezometers, by the mechanical squeezing or leaching of clay cores in the laboratory. At this moment, pore water obtained from piezometers is considered the most representative for the in situ pore water. Although there is already detailed information about various aspects of Boom Clay and geological disposal, our current understanding of the impact of microbial metabolism on a repository system remains tenuous. It is clear that microorganisms are ubiquitously present in Boom Clay pore water, however, it remains unclear how they affect the geochemical environment and hence ultimately radionuclide migration. Moreover, conditions in the repository are far from optimal (e.g. high temperature, high pH, space restriction), however current projects have been shown that temperature and space restriction alone are not completely abolishing microbial presence and/or activity. But more detailed studies are necessary.
The main goal of this project is to assess the presence of microbial communities present in the HADES underground research laboratory and to examine their impact on the geochemical conditions of the pore water. Moreover, the impact of conditions distinctive to geological disposal of radioactive waste on these communities will be investigated in detail. In addition to the bacterial population, the focus will be on the archaeal community present in this environment as a clear methane production was observed in at least one of the piezometers from HADES. For each of the piezometers, a detailed physico-chemical analysis is performed, enabling to correlate the observed microbial population within a specific piezometer to the chemical conditions.
In first instance, the microbial community will be determined using a global metagenomic approach. The overall microbial populations will be assessed using a 16S rRNA amplicon sequencing approach. By selectively sequencing marker genes like 16S rRNA (or mcrA), the type and relative abundance of microorganisms (and methanogenic archaea) can be identified. For the most interesting samples, the complete metabolic potential of the microbial community will be identified using a shotgun metagenomics approach (i.e. not only sequencing a marker gene, but rather sequence all available DNA). By additionally implementing a metaproteomics approach, it will also be possible to identify those parts of the microbial genomes essential for surviving in those environments.
In addition, the microbial community will be exposed to different stressors such as high pH, temperature, radiation, ionic strength, ... to elucidate the impact on the microbial community and on pore water composition. Furthermore, these conditions will be used to isolate individual bacterial and archaeal strains from the community. In this way, new species could be identified that will be subjected to whole-genome sequencing, followed by genome annotation. Additionally, these isolates will be phenotypically characterized in detail.
This study will give more insights in the microbial communities present in Boom Clay borehole water, their impact on the pore water chemistry and their response to conditions distinctive for radioactive waste disposal.