Microbiology & dissolved gases

Cyclic changes in temperature induced by the operation of the HT-BTES facility are also expected to alter the microbial activity in the subsoil. In particular, a moderate increase in temperature is expected to promote microbial growth rates, which should be reflected in the amount of microbial respiration within the aquifer. This could lead to both a decrease in the amount of dissolved oxygen in the groundwater and an increase in the released carbon dioxide. Increased microbial growth could also result in enhanced biofilm formation, which could potentially clog some of the soil and rock interstices and thus alter locally the aquifer flow properties.

The aforementioned changes are evaluated through periodic sampling campaigns, in which biological material for later analyses is obtained by filtering the groundwater extracted from different depths. From the obtained samples, we first assess the microbiome by flow cytometry to obtain the microbial cell concentrations present in the aquifer. In addition, DNA is extracted and sequenced to identify and quantify the individual microbial taxa and observe how they change in response to the changing conditions.

To address the open questions regarding bacterial respiration and the impact of temperature cyclic fluctuations on these systems, we also monitor the concentration of dissolved gases in the groundwater. This is achieved using a portable mass-spectrometer (miniRUEDI by Gasometrix), which continuously measures the concentration of CO₂, CH₄, O₂, among other gases, in the groundwater pumped out from all sampling locations. This information supports our assessment of the groundwater chemical composition and provides insights into the interplay between temperature changes, groundwater chemical properties, and microbial activity. This will further help us to conclude on the most optimal conditions for the future operation of HT-BTES facilities.