Boom Clay pore water analysis shows the presence of an active methanogenic microbial community

In Belgium, Boom Clay is considered a potential host rock for the geological disposal of radioactive waste. Geological disposal of radioactive waste requires a detailed understanding of the geochemical conditions present in the host rock. To this end, analysis of pore water is essential, as its composition determines among others, the speciation and solubility of radionuclides. Although the elemental composition of Boom Clay pore water is relatively well known, the real mechanisms controlling the pCO2 (g) and the pH are not yet completely understood. Currently these parameters are mostly calculated, however, these models are not yet validated with in situ measurements. Microorganisms have been shown to be present in many borehole waters of different URL’s and are known to be able to affect the geochemistry of the environment. However, the possible impact of microorganisms on the geochemistry of Boom Clay pore water is not yet examined. This study describes and discusses the evolution of the geochemistry and the microbial community in the pore water from the piezometers from the PRACLAY gallery from the installation until operation, before the start of the PRACLAY Heater test. Overall, most geochemical parameters seem to vary during the first 4 years (from 2007 – 2011), while afterwards (2011-2013) they remain quite stable. However, pCO2 values decrease over time concomitantly with an increase in pCH4. This reduction can be explained by the presence of a complex methane producing microbial community in the pore water of the piezometers from the PRACLAY gallery. Moreover, different experimental interventions probably stimulated the present microbial community, resulting in increased methane production rates when conditions were more favourable again for methanogenesis. The presence of an active microbial community can explain why experimental pCO2 – pH data do not correspond to the data obtained by predictive modelling, hampering proper modelling of the ongoing geochemical processes. Detailed knowledge on the reaction mechanisms and kinetics can improve the current predictive models, therefore more detailed microbial and geochemical monitoring should be performed in future studies.